Color selective light modulators employing birefringent stacks

ABSTRACT

The present invention provides a high brightness subtractive color filter formed by an electro-optic modulator positioned between two retarder stacks. The second retarder stack echoes the first retarder stack, having the same sequence of retardances but in reverse order and having an orientation rotated with respect to the first stack. The modulator changes the apparent orientation of the second stack so that, in a first switching state of the modulator the two stacks cooperate in filtering the spectrum of input light, and in a second switching state they vanish, leaving white light. Two or more stages can be used in series, each stage providing independent analog control of a primary color. One preferred embodiment eliminates internal polarizers between stages, thereby providing a full-color display with only two neutral polarizers. Hybrid filters can be made using the filter of this invention in combination with other active or passive filters. The color filters of this invention can be used in many applications, particularly in the areas of recording and displaying color images. They can be arranged in a multi pixel array and can be optically addressed.

This application is a Continuation-In-Part Application of U.S. Ser. No.08/645,580 filed May 14, 1996, now U.S. Pat. No. 5,822,021, the contentsof which are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to color selective polarization modulation and toutilization thereof in filters having subtractive color systems and todevices such as cameras and displays utilizing the color filter.

2. Background of the Related Art

Color display is generally provided by spatial or temporal multiplexingof the additive primary colors, red, green and blue. In a spatialmultiplexed display, each color pixel is divided into three subpixels,one for each primary color. Ideally the pixels are small enough comparedto the viewing distance from the eye that the colors are spatiallyintegrated into a single full-color image. As a result of subdividingeach pixel, the spatial resolution of the display is reduced by a factorof at least three. In temporal multiplexing, colors are sequentiallyswitched between the three primary colors, and if the switching rate isfast enough the eye temporally integrates the three images to form asingle full-color image. In both cases, the color filter is typicallycombined in series with a binary or display capable of generating a grayscale which is spatially aligned and temporally synchronized with thecolor filter to modulate the intensity of each color. To display whitewith spatial multiplexing, all three subpixels simultaneously transmit aprimary; with temporal multiplexing the three primaries are sequentiallytransmitted. In either case, at best only one third of the inputintensity can be displayed.

In subtractive display, color is produced by stacking three monochromedisplays (for example Plummer, U.S. Pat. No. 4,416,514 and Conner etal., U.S. Pat. No. 5,124,818). Polarization components are placedbetween each display panel, such that each panel ideally independentlycontrols the transmission of an additive primary color. Subtractivedisplays have the advantage that every pixel is a three-color pixel andthat the display does not, in principle, suffer the throughput lossassociated with spatial or temporal multiplexing. However, previousimplementations generally could not completely independently modulateeach color. Additionally, they utilized pleochroic dye polarizers as theonly color selective polarization components between each display panel.Due to the poor performance of pleochroic dye polarizers, including poorcolor contrast, high insertion loss and shallow transition slopes, thebenefits of subtractive displays have not before been realized.

The above references are incorporated by reference herein whereappropriate for appropriate teachings of additional or alternativedetails, features and/or technical background.

SUMMARY OF THE INVENTION

The present invention provides a color selective polarization modulatorand a high brightness color filter or display system. Color separationis accomplished with nearly lossless retarder films, providing highcolor contrast between transmission and extinction, with steeptransition slopes. Each filter stage is a color selective light valvewhich varies the transmission (or reflection) of one color withoutmodulating the complementary color. A stage can switch betweentransmitting white (or black) and transmitting a filtered spectrum. Twoor more stages can be used in series, each stage independentlycontrolling the transmission of a primary color. In a preferredembodiment each stage can control the analog intensity control of theprimary color at each pixel, thus eliminating the need for an externalgray-scale pixelated display. One preferred embodiment eliminatesinternal polarizers between stages, thereby providing a full-colordisplay with only an input polarizing means and an output polarizingmeans.

The color selective polarization modulator can be for example anelectro-optic or magneto-optic modulator having a modulation state ofpolarization and an isotropic state of polarization, and a retarderstack comprising one or more retarders. The modulation state ofpolarization is an input polarization for which the transmitted state ofpolarization depends on the voltage applied to the modulator. Theisotropic state of polarization is an input polarization for which thetransmitted state of polarization is substantially independent of thevoltage applied to the modulator. The retarder stack chromaticallypreconditions the light such that a first spectrum is placed in themodulation state of the modulator and a second, complementary, spectrumis placed in the isotropic state. The modulator thereby modulates thestate of polarization of the first spectrum, but leaves the polarizationof the complementary spectrum substantially unmodulated. In a preferredembodiment the spectra are additive and subtractive primary spectra.

A filter is formed by combining the color selective polarizationmodulator with a polarization analyzer. The polarization analyzer can bea second retarder stack in combination with a neutral polarizer, or itcan be a color selective polarizer such as a linear or circular coloredpolarizing filter, examples of which are pleochroic dye polarizers, andcholesteric liquid crystals, and cholesteric liquid crystal polymers,respectively.

For the case where the polarization analyzer is a second retarder stackin combination with a neutral polarizer, the second retarder stackechoes the first retarder stack, having the same sequence of retardancesbut in reverse order. The orientation of the second stack is alsorotated with respect to the first stack. As a result, in one switchingstate of the modulator the second stack appears to be crossed with thefirst stack, undoing the polarization transformation caused by the firststack and, for parallel input and output polarizing means, transmittingwhite light. For crossed polarizers, the transmission is black. In asecond switching state the two stacks are seen as a unit in which thesecond retarder stack completes the transformation started by the firststack and orthogonally polarizes the first and second spectra. In thisstate the filter transmits a filtered spectrum.

In the two-stack filter, the polarizers and stacks can be oriented sothat the filter is either normally white, i.e. white in the absence ofthe modulator, or normally filtered. In the former, the action of themodulator is to produce a filtered output, while the latter uses themodulator to generate the white state. In either case, the voltageapplied to the modulator controls the "presence" of the compound stack,i.e. the extent to which the two stacks cooperate rather than canceling.If the modulator is capable of analog modulation, the voltage controlledpresence of the compound stack is also analog. Analog control of thevoltage produces variable throughput of the filtered spectrum.

Each retarder stack has one or more retarders. In order for two stacksto cancel one another in one switching state, if the first stack ofretarders have retardances Γ₁, Γ₂ . . . Γ_(N) and orientations α₁, α₂ .. . α_(N), then the second retarder stack has retardances Γ_(N) . . .Γ₂, Γ₁ and orientations 90±α_(N) . . . 90±α₂, 90±α₁. For parallelpolarizers the filter is normally white when the second stack retardersare oriented at 90+α_(N), and it is normally filtered when the secondstack retarders are oriented at 90-α_(N).

Suitable two-stack designs can be generated by choosing the number ofretarders N, stepping through a range of values for Γ_(N) and α_(N),applying the above rules of retarder orientation to define the secondstack, calculating the transmission of the filtered spectrum, andselecting filter designs that produce the desired spectra, typicallyadditive or subtractive primary spectra. Alternatively, certain classesof filter designs can be employed which lend themselves to thewhite/filtered structure. In particular fan and folded olc filters canbe adapted to fit the orientation requirements, as can split-elementfilters.

In addition to the retarder stacks, additional polarization transformingelements can be included between the input and exit polarizers, forexample to resolve compatibility issues between the polarizers and thetype of modulator. For polarized light sources, no input polarizer isrequired. In embodiments having no internal polarizers, the filters canbe operated in polarization diversity configurations having polarizationseparators/combiners for the input and exit polarizers. The filters canalso employ reflection-mode designs.

Hybrid filters can be made using the filter of this invention incombination with other active or passive filters. The color filter ofthis invention can be combined with passive filters, such as retarderbased notch filters and dichroic filters for blocking UV, IR or otherbands of light. It can be used with other active filters, such aspolarization interference filters and switched polarizer filters.

The spectral filters of this invention are particularly useful in thevisible spectrum as color filters. They can also be fabricated for usein other wavelength bands for spectroscopy, spectrometry night visionfiltering, or wavelength division multiplexing applications. The colorfilters of this invention can be used in many applications, particularlyin the areas of recording and displaying color images. They can bearranged in a multi pixel array, can be spatially or temporallymultiplexed, and can be optically addressed.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objects and advantages of the invention may be realizedand attained as particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to thefollowing drawings in which like reference numerals refer to likeelements wherein:

FIG. 1 shows a filter using a color selective polarization modulator incombination with a polarization analyzer.

Polarizer 40 can be a holographic polarizer which is a hologram thatsees one polarization state and not the other. Holographic polarizerdefracts light of a first state of polarlization and does not defractlight of another state of polarization. Polarizer 40 can also be an EMIndustries Transmax polarizer.

FIG. 2, comprising FIGS. 2a-b, is a filter, in which the polarizationanalyzer is a retarder stack in combination with a neutral polarizer,transmitting (a) white in a first switching state of the modulator and(b) a filtered spectrum in a second switching state.

FIG. 3, comprising FIGS. 3a-b, is a filter, in which the polarizationanalyzer is a linear colored polarizing filter, transmitting (a) whitein a first switching state of the modulator and (b) yellow in a secondswitching state.

FIG. 4, comprising FIGS. 4a-b, is a filter, in which the polarizationanalyzer is a cholesteric liquid crystal, transmitting (a) white in afirst switching state of the modulator and (b) yellow in a secondswitching state.

FIG. 5 is a normally white embodiment of the FIG. 2 filter.

FIG. 6 is a normally filtered embodiment of the FIG. 2 filter.

FIG. 7, comprising FIGS. 7a-d, show filters with a total of (a) tworetarders and (b-d) four retarders.

FIG. 8 is the measured transmission of a G/W filter.

FIG. 9 is the measured transmission of a W/C filter.

FIG. 10 is the measured transmission of a W/M filter.

FIG. 11 is the measured transmission of a W/Y filter.

FIG. 12 shows the measured transmission of the W/M filter of FIG. 10 asa function of incidence angle for an azimuth angle of 0°.

FIG. 13 shows the measured transmission of the W/M filter of FIG. 10 asa function of incidence angle for an azimuth angle of 90°.

FIG. 14 shows the measured transmission of the W/M filter of FIG. 10 asa function of rms voltage applied to the LCD.

FIG. 15 shows the continuous modulation of a white/cyan filter stage.

FIG. 16 shows the continuous modulation of a white/magenta filter stage.

FIG. 17 shows the continuous modulation of a white/yellow filter stage.

FIG. 18 shows the extreme switching states of a magenta/white filterstage.

FIG. 19 is a multiple stage filter with polarizers between stages.

FIG. 20 is a filter comprised of blue, green and red modulating stageswith no internal polarizers.

FIG. 21 is a specific three stage filter design.

FIG. 22, comprising FIGS. 22a-c, is the (a) red, (b) green and © blueoutputs of the filter of FIG. 21.

FIG. 23, comprising FIGS. 23a-c, is the (a) cyan, (b) yellow and ©magenta outputs of the filter of FIG. 21.

FIG. 24 is white, black and gray scale output of the filter of FIG. 21.

FIG. 25, comprising FIGS. 25a-c, shows (a) red, green and bluetransmission spectra of a three stage filter, and the dark statesachieved with (b) polarizers between stages and © no internalpolarizers.

FIG. 26 is a polarization independent multistage filter.

FIG. 27 is a filter having nested polarization stacks.

FIG. 28 is a three stage filter employing cholesteric liquid crystalsfor the polarization analyzers.

FIG. 29 shows a stack which includes a first and second retarder havinga fist and second orientation, as well as a first and second retardance.

FIG. 30, comprising FIGS. 30a and 30b shows two general examples ofpartially polarized light input to stacks corresponding to the stack ofFIG. 29.

FIG. 31, comprising FIGS. 31a and 31b shows stacks with the addition ofpolarizers.

FIG. 32, comprising FIGS. 32a and 32b shows a device for manipulatingpartially polarized light and outputting resulting modulator outputlight.

FIG. 33a shows a device for manipulating at least partially polarizedlight and FIG. 33b corresponds to FIG. 33a for the case in whichpartially polarized light is elliptically polarized.

FIG. 34 is a subtractive color filter of the prior art.

FIG. 35, comprising FIGS. 35a and 35b, is a filter having a twistednematic electro-optic modulator.

FIG. 36 is a three stage filter using the filter of FIG. 35.

FIG. 37 shows an in-line filter using a first stage to temporallymultiplex two primary colors with a second stage dedicated to the thirdprimary color.

FIG. 38 shows an example of a passive prefiltering device where theboxes indicate retarders oriented at the angles shown in the bottom partof the box.

FIG. 39 shows the transmission for visible light incident upon thisfilter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The color selective polarization modulator of this invention isillustrated in FIG. 1. Polarization modulator 60 is formed by modulator10 in combination with retarder stack 20. The polarization modulatoruses polarized input light, in this case polarization P_(o) is providedby polarizer 40. The polarization modulator can be combined withpolarization analyzer 70 to form a filter.

As shown in FIG. 1, incident white light can be considered to be thecombination of light with spectrum F and light with the complementaryspectrum F. Retarder stack 20 transforms the polarization of light inspectrum F into the modulation state of polarization, P_(M), ofmodulator 10, and transforms light in spectrum F into the isotropicstate of polarization, P_(I). For one of the spectra the transformationcan be an identity transformation, i.e. the polarization can beunchanged. Light of polarization P_(I) is transmitted by modulator 10with a polarization which does not vary with the voltage applied. Lightwith polarization P_(M) is modulated as a function of the appliedvoltage into polarization P_(M). In this way the polarization of oneportion of the input light is modulated and the rest is not.

The modulator is a device which controls the state of polarization oftransmitted light with the application of a voltage. The modulator hasboth a modulation state of polarization and an isotropic state ofpolarization, i.e. there is a state of polarization of input light forwhich the transmitted state of polarization depends on the voltageapplied to the modulator, and a state of polarization for which it issubstantially independent thereof. For the isotropic state typically thepolarization is unchanged, but it some systems the polarization can havea voltage-independent transformation such as 90° rotation of linearlight or handedness reversal of circular light. The terms polarizationand state of polarization are used interchangeably.

One class of suitable modulators is the electro-optic variable retarderhaving fixed orientation and voltage controlled retardance. Theisotropic states of polarization are parallel and perpendicular to theplane of the optic axis. In this case they are the Eigenmodes, orordinary and extraordinary waves, of the retarder. There is a voltagecontrolled phase shift between the ordinary and extraordinarypolarizations. This does not affect the transmitted polarization ineither of the isotropic states. However, the transmitted polarization ischanged when the input light has projections in both isotropic states.The preferred modulation state is the linear polarization that bisectsthe two isotropic polarizations. It has maximum modulation depth becausethe projections along the two isotropic states have equal amplitude.Electro-optic modulators including LiNbO₃, quartz, and liquid crystalssuch as, zero-twist nematics, including homogeneously alignedelectrically controlled birefringence (ECB), homeotropically alignedoptically controlled birefringence (OCB), hybrid aligned nematic (IAN),and pi-cell/surface mode nematics, are one of the preferred embodimentsof this group of electro-optic modulators.

Optically active devices are another group of suitable electro-opticmodulators. Optically active devices are polarization rotators, with therotation being independent of the orientation of the polarization of theincident light. The isotropic states are left and right handed circularpolarizations. The modulator performs voltage controlled phase shiftingof the circular polarizations, which does not affect the circular stateof polarization. However, for linear states, which can be decomposedinto equal amplitudes of the circular polarizations, the transmittedpolarization is linear and the orientation is determined by the phaseshift between the circular states. Thus optically active modulators actas polarization rotators for linear states, and the modulation state ofpolarization is any linear state.

Chiral Smectic Liquid Crystal (CSLC) retarders reflect polarizationthrough an axis rather than rotating polarization, but they are likeoptically active devices in their isotropic and modulation states ofpolarization. In contrast to zero-twist nematics, CSLCs are rotatableretarders having fixed retardance and optic axis orientation determinedby the applied voltage. For a half-wave retardance, CSLCs have circularisotropic states and linear modulation states. The voltage controlledorientation of the optic axis of the CSLC determines the transmittedorientation of linear light.

There are also electro-optic modulators that have intermediatemodulation and isotropic states, such as twisted nematic devices. It hasbeen shown that a twisted nematic device is an elliptical retarder withelliptical Eigenpolarization states. Like the linear retarder, the phaseshift is substantially determined by one particular helicity ofpolarization state. (J. L. Pezzaniti and R. A. Chipman, (1993),"Phase-only modulation of a twisted nematic liquid-crystal TV by use ofthe eigenpolarization states," Opt. Lett. 18, 1567-1569.)

Suitable nematic liquid crystal cells for use in the electro-opticmodulator include twisted nematic (TN), super twist nematic (STN),electrically controlled birefringence, hybrid field effect, pi-cell andsurface mode, zero-twist mode, hybrid aligned nematic liquid crystalretarders. Suitable smectic liquid crystal cells include chiral smectic,ferroelectric, SmC*, surface stabilized SmC*, volume stabilized SmC*,binary SmC*, analog SmC*, SmA*, electroclinic, distorted helixferroelectric, anti-ferroelectric, flexoelectric, and achiralferroelectric liquid crystal retarders. If lateral electrodes on one ofthe device substrates are used rather than transparent electrodesdeposited on opposing device substrates, nematic liquid crystals canoperate as rotatable retarders with fixed retardance, and smectic liquidcrystal can operate as variable retarders with fixed orientation.

Compound retarders utilizing a liquid crystal active retarder incombination with one or more passive retarders can be used in theelectro-optic modulator. Particularly useful are achromatic compoundretarders as described in U.S. patent application Ser. No. 08/419,593,and achromatic polarization rotators as described in U.S. patentapplication Ser. No. 08/549,963, both of which are incorporated byreference in their entirety herein. The achromatic compound retardercomprises a liquid crystal rotatable half-wave retarder flanked bypassive retarders, wherein the orientations and retardances of thepassive retarders are such that the compound retarder is achromatic. Theachromatic polarization rotator comprises a liquid crystal rotatablehalf-wave retarder in combination with a passive half-wave retarder.

For compatibility between the retarder stacks and the electro-opticmodulator, passive retarders can be included in the electro-opticmodulator. For example, if the retarder stacks prepare the light in twolinear polarizations separated by 45°, these are the modulation andisotropic states of polarization for a variable retarder with fixedorientation. A passive quarter-wave retarder oriented parallel to one ofthe polarization states converts the two linear polarizations into onelinear and one circular polarization, which are the modulation andisotropic states of polarization for a rotatable retarder with fixedretardance. Thus if the electro-optic modulator includes a passivequarter-wave retarders on either side of a CSLC, stacks designed for avariable retarder can instead be used with a rotatable retarder.

Retarder stack 20 includes one or more passive retarders. For Nretarders, the orientations are α₁ through α_(N) and the retardances areΓ₁ through Γ_(N). Any retardation material can be employed in theretarder stacks. Retarder materials preferably provide the following:high optical clarity, uniform retardance, range in retardance sufficientfor the design requirements (this depends upon range of inducedbirefringence and practical range in thickness), environmentaldurability, and in many cases large area and low cost.

Retarder stacks can be constructed using, for example, layers ofform-birefringence devices, liquid crystal polymer films, stretchedpolymer retarder sheets, or crystalline retarders. Stretched polymerfilms are available in arbitrary retardances (0-2,000 nm), using avariety of materials with unique birefringence dispersioncharacteristics. Large sheets can be purchased at a low cost, permittinglarge clear aperture filters. The characteristics of z-stretchedpolymers (Nitto NRZ) permit large view angles with small retardanceshifts. Several other polymer materials are useful in producing filters,including but not limited to, poly-vinyl alcohol, polycarbonate, mylar,polypropylene, polystyrene, triacetate (tri-butyl-acetate), andpolymethylmethacrylate.

Liquid crystal polymer films, particularly UV cross-linkable polymernematic linear retarders, are particularly suitable for forming retarderstacks. An attractive feature is the ability to produce thin high-orderretarders, since the material can have very high birefringence. This canpermit the fabrication of multi-layer stacks on a single substrate withlow cost. Liquid crystal polymers are particularly well suited tofabricating retarder stacks containing two or more retarders. Tofabricate liquid crystal polymer layers, an alignment layer is firstdeposited on the substrate and then photopolymerized with polarizedlight. The orientation of the polarized light governs the orientation ofthe alignment layer and the subsequently deposited liquid crystalpolymer layer. Since the polarization of the photopolymerizing light isreadily controlled, stacks of liquid crystal polymer retarders can bemanufactured with controlled alignment of each retarder in the stack.This is particularly advantageous for oblique relative orientations,i.e. relative orientations other than 0 or 90°, for which alignment ofsheet retarders is more difficult.

Conventional crystalline retarder materials, such as quartz, mica andcalcite, are well suited to applications requiring higher resolutionthan is feasible with polymer films. They are also useful forapplications requiring low wavefront distortion, and/or high powerhandling requirements. They are more expensive than polymer retardersand do not lend themselves to large area, particularly when lowretardances are required.

Using Poincare sphere analysis, suitable orientations for the retarderswere calculated for the case wherein the modulator has isotropic stateswhich are linear polarizations at 0 or 90° and modulation states whichare linearly polarized at ±45° (eg. a zero twist nematic, ZTN). If allthe retarders have the same retardance the orientations are related by,

    α.sub.N -α.sub.N-1 + . . . α.sub.2 -α.sub.1 =π/8+πm/4                                           Eq. 1

where m is an integer. For example, if there is one retarder, it can beoriented at π/8, 3π/8, 5π/8, . . . for m=0, 1, 2, . . . . For tworetarders, once α₁ is chosen, α₂ can be determined. For example, for α₁=π/16, Eq. 1 gives α₂ =3π/16+πm/4, which is 3π/16 for m=0. Theorientations can similarly be determined for more that two retarders.Poincare analysis can also be used to calculate retarder orientationsfor other choices of isotropic and modulation polarizations.

Combining polarization modulator 60 with polarization analyzer 70creates a filter which intensity modulates the first spectrum, having anoutput labeled % F in FIG. 1, but has a constant output for spectrum F.Depending on the analyzer, the output of F is fixed somewhere between 0and 100%. The polarization analyzer can be a second retarder stack incombination with a polarizer. A second retarder stack is not requiredwhen the analyzing polarizer is isotropic to spectrum F. Thus linear orcircular colored polarizing filters can be used as the polarizationanalyzer.

A filter wherein the polarization analyzer includes a second retarderstack is illustrated in FIG. 2. It is formed by modulator 10 positionedbetween first retarder stack 20 and second retarder stack 30. Themodulator changes the apparent orientation of the second stack so that,in one switching state of the modulator the two stacks cooperate infiltering the spectrum of input light, and in another switching statethe retarders essentially vanish, resulting in an unfiltered or whiteoutput spectrum (FIG. 2a).

The filter includes input polarizer 40 and analyzing polarizer 50 whichcan be neutral polarizers. Orientation angles are defined with respectto the polarization of input light, in this case defined by polarizer40. If the light source provides polarized light, an input polarizer isnot required. The filter can be coupled with a separate polarizingdevice which can serve as the analyzing polarizer. In this embodimentthe filter is illustrated with linear polarizers. In general they can belinear, circular or elliptical polarizers. Suitable polarizers includeabsorption, dichroic, dye-based, non-absorptive, polarizing dielectricfilm, polarizing beamsplitter, calcite, quartz, scattering, prismatic,cholesteric and stacked cholesteric polarizers. The polarizer caninclude additional polarization conditioning elements, such as aquarter-wave retarder to convert between linear and circularpolarization.

The filter is illustrated with a zero-twist nematic modulator switchedbetween (a) half-wave retardance and (b) zero retardance. Retarders arerepresented by boxes with the retardance labeled at the top and theorientation with respect to the input polarization, in degrees, at thebottom. The retarders have the specified retardance at a designwavelength, which is typically within the operating range of the filterand which can be tailored to optimize filter performance. Allorientations given herein are approximate and can be adjusted by severaldegrees to tailor the filter output.

In the filter of FIG. 2, modulator 10 is an electro-optic, zero-twistnematic liquid crystal oriented at 0° with retardance which variesbetween λ/2 when no field is applied ("off") and ideally zero when thefully "on" field is applied. For this modulator the isotropic states arelinear polarizations at 0 or 90° and the modulation states are linearlypolarized at plus and minus 45°. In this embodiment the modulator is aliquid crystal display (LCD), where the term LCD is used for any liquidcrystal device which contains a liquid crystal cell having one or morepixels. The LCD is typically a multi pixel array of liquid crystal cellswhere each pixel can be independently controlled. The filters can beimplemented with a multi pixel LCD or a nonpixelated electro-opticmodulator. The retardances of a single pixel of an LCD are labeled inFIG. 2

In the illustrated embodiment white light is incident on polarizer 40.For filters designed to operate outside of the visible spectrum the"white" light is all of the input wavelengths of light, for example ininfrared wavelength division multiplexing it would be all the infraredchannel wavelengths. The white light can be thought of as being composedof a first spectrum labeled F, and a second, complementary, spectrumlabeled F.

The retarders in stack 20 have retardances and orientations such thatspectrum F is output or transmitted in the modulation state of 45°linear polarization and spectrum F is output or transmitted in theisotropic state of 0° linear polarization. In a first switching state(FIG. 2a) the modulator has ideally retardance λ/2, whereby thepolarization of the spectrum seeing the modulation state is switched to-45°. The isotropic state is unchanged. In a second switching state(FIG. 2b) the modulator has zero retardance and the polarization of thelight output from the modulator is unchanged for both the modulation andisotropic states.

Second retarder stack 30 follows the modulator. When the modulator is inthe first switching state the second stack undoes the transformation ofthe first stack, thereby transmitting both F and F linearly polarized at0°. Polarizer 50 transmits both spectra and the filter output is whitelight. In the second switching state, the second stack completes thetransformation which orthogonally polarizes F and F. Polarizer 50 blocksthe light having spectrum F, and the output is filtered light withspectrum F. If polarizer 50 were oriented at 90°, the filter wouldswitch between black and the F spectra, the complements of white and F.Because the output is filtered in the second state, wherein themodulator is not "seen", the filter is called normally filtered (NF). Afilter that transmits (or blocks) the entire spectrum with the LCDremoved is called normally white (NW). In general, the normal state islisted first in naming a primary/white or white/primary filter, but theterm white/primary is also used generically for both.

In a preferred embodiment the filter is a color filter and spectra F andF correspond to an additive primary (red, green or blue) and thecomplementary subtractive primary (cyan, magenta or yellow). Either theF or the F spectrum can be the additive primary. In the visible, thefilter is therefore a primary/white filter for parallel polarizers, or aprimary/black filter for crossed polarizers. The filter is generallydescribed herein for the case of color filtering, though in general itcan be used as a white/filtered switch wherein the filtered spectrum isnot limited to a primary color spectrum. The filter is named for thespectrum it transmits rather than the spectrum it modulates. Thus thefilter of FIG. 2 is a F/W filter, even though it is the F spectrum whichis modulated by for example, an electro-optic modulator.

Polymers are chemical compounds or mixtures of compounds consisting ofrepeating structural units formed by a chemical reaction where two ormore small molecules combine to form larger molecules.

Liquid Crystal Polymers (LCP) are a class of polymers wherein liquidcrystal monomers are incorporated into the macromolecular structurealong the mainchain (backbone) or as side chain units.

LCP's can be aligned by either mechanically rubbed surfaces, shearing,or more recently it has been shown by optical means. Optical methodsinclude first applying linear photo-polymerizable (LPP) films orazo-based dyes either in a polymer alignment layer. In the former (seeSchadt, et al, Jpn. J. Appl. Phys. Vol. 34, pg. 3240-3249, 1995), theLPP materials are deposited on a substrate and then cured at elevatedtemperature. The cured film is then subjected to polarized UV light.LCP's are then spun-on or coated onto the same substrate, and align withthe orientation of the LPP film. The LCP's are then cross-linked byexposure to unpolarized UV light. In the latter, (Shannon et al, Nature,vol. 368, pg. 532-533, 1994), azo-dye molecules intercalated into thepolymide alignment layer, (or layers) which are deposited onto varioussubstrates (including glass, silicon, and others). A liquid crystallinemonomer or polymer is either deposited onto one substrate, or sandwichedin between two substrates. The LC molecular director orientsperpendicular to the direction of the polarized UV light whichpreviously illuminated the alignment layer. Subsequent exposure willreorient the liquid crystals, which may be disadvantageous for someapplications.

FIG. 2 illustrates the two extreme switching states of the modulator. Ifthe LCD pixel has a retardance other than 0 or λ/2, the F spectrum stillretains the 0° polarization orientation and is fully transmitted.However light in the F spectrum is transformed to an ellipticalpolarization having a projection in the 0° polarization state which isbetween 0% (FIG. 2b) and 100% (FIG. 2a). Analog control of the modulatorvoltage therefore provides analog modulation of the F spectrum.

A Color Selective Polarizing (CSP) filter for example, a pleochroic dyepolarizer, for the analyzing polarizer is illustrated in FIG. 3. Lightpropagation is illustrated with arrows having the color labeled on topand the orientation on the bottom. In the specific embodimentillustrated, retarder stack 20 prepares blue light at 0° and yellowlight at 45°. Electro-optic modulator 10 is a ZTN oriented at 45°, forwhich a linear polarization at 0° is modulated and a linear polarizationat 45° is isotropic. Analyzing polarizer 80 is a yellow (CSP) orientedat 90°, which transmits yellow light at 0° and all wavelengths at 90°.When the ZTN has half-wave retardance (FIG. 3a), the blue light isrotated to 90° and is transmitted by the (CSP) filter. When the ZTN haszero retardance (FIG. 3b), the blue light remains at 0° and istransmitted by the (CSP). For intermediate retardances, the bluetransmission is varied between 0 and 100%.

A filter having a circular color polarizer, in this case a cholestericliquid crystal (CLC) or CLC Polymer, is illustrated in FIG. 4. Retarderstack 20 transmits blue light at 45° and yellow light at 0°.Electro-optic modulator 10 is a ZTN oriented at 0° and having retardanceswitchable between 3λ/4 (FIG. 4a) and λ/4 (FIG. 4b). For the blue lightat 45°, the polarization is therefore modulated between left-handed andright-handed circular polarizations. For intermediate retardances,intermediate elliptical polarizations are produced. Component 90 is aright-handed blue CLC, which reflects right-handed blue light, transmitsleft-handed blue light, and transmits other wavelengths regardless ofpolarization. Because the CLC reflects rather than absorbing thenon-transmitted polarization, this filter provides variable blue outputboth in transmission and reflection mode.

The remainder of the description is addressed primarily to the tworetarder stack filter, although much of it can be generalized to anypolarization analyzer. In order for the second stack to undo thetransformation of the first stack, the retarders of the two stacks arerelated to each other as shown in FIG. 5 for a normally white filter andin FIG. 6 for a normally filtered filter. In both figures modulator 10is shown in the switching state in which it is not seen, so that itillustrates the normal transmission of the filter. The first stack hasretarders 21, 22 and 23 having retardances Γ₁, Γ₂ . . . Γ_(N) andorientations α₁, α₂ . . . α_(N). For equal retardances, the orientationsare related according to Eq. 1. The second stack contains retarders33a,b, 32a,b and 31a,b having the same retardances as the first stackbut with the order reversed. The retarders of the second stack arerotated with respect to the first stack to orientations of 90+α_(N) forNW and 90-α_(N) for NF. In the NW configuration, the Jones matrices forthe first and second stacks are inverses of one another.

When modulator 10 is switched to half-wave retardance, the NW structureprovides a filtered spectrum and the NF structure provides a whitespectrum. If the LCD were completely achromatic, the filter of FIG. 5would, in the switched state, provide the same spectrum as the FIG. 6filter, and vice versa. The main difference between the NW and NFconfigurations is a result of the chromatic effects of the LCD. In theNW configuration the white state is ideal, but the chromaticity of theLCD can degrade the color contrast of the filtered state. For the NWsubtractive white/primary filters (W/C, W/M or W/Y) the LCD need only beachromatic over the bandwidth of the modulated additive primary (R, G orB). Conversely, NW designs for W/R, W/G or W/B filters give best resultwhen the LCD is achromatic over the bandwidth of the modulatedsubtractive primary. The LCD is preferably optimized for switching ofthe modulated band, for example by having the LCD design wavelengthwithin the modulated band. This is most challenging for normally whiteW/G filters, in which the modulated band is the subtractive primarymagenta, which contains both red and blue bands.

In NF configurations the white state can suffer from chromaticity.However, since at least one additive primary band is always transmittedregardless of the LCD retardance, the LCD retardance can be selected tooptimally pass the modulated subtractive primary band. Chromaticity inthe white state is less detrimental than in the filtered state becausethe resulting loss of throughput in the white state impacts colorquality less than leakage in the filtered state. Thus, in terms ofchromaticity, a G/W filter can perform better than a W/G filter.

Useful stack designs can be generated by starting from filter designs,such as olc or split-element filters, having structures which happen tonaturally fit the constraints. Alternatively, filter designs havinguseful transmission spectra can be generated, for example using thenetwork synthesis technique (see Harris et al. (1964), J. Opt. Soc. Am.54:1267; Ammann et al. (1966), J. Opt. Soc. Am. 56:1746; Ammann (1966),J. Opt. Soc. Am. 56:943; and U.S. patent application Ser. No.08/447,522, filed May 23, 1995, all of which are incorporated byreference herein in their entirety), and then screened for the subsetconforming to the design requirements. A third approach is tosystematically evaluate all the designs that fit the design requirementsto locate those with useful spectral profiles.

This third approach is illustrated in FIG. 7. The illustrated filtersfollowing the NF design requirements of FIG. 6. The LCD modulators areswitchable between two retardances; both are listed and are separated bya comma. For analog modulators, the retardance is continuously tunablebetween these extreme values. FIG. 7a shows the simplest filter design,having only one retarder in each of stacks 20 and 30, for a total of tworetarders. FIGS. 7b-d show designs having a total of four retarderseach. For the special case where α₂ =45°, in the normal state it isequivalent to a three retarder filter having a center retarder withretardance 2Γ₂ and orientation 45°.

In the illustrated filters, the retarders in each stack have eitherequal retardances or retardances differing by a factor of two, therebyproviding real impulse response functions. This is not a requirement,the retardances can be unrelated, resulting in complex impulse responsefunctions. Equal retardances can facilitate fabrication. These examplesare by no means a complete set of useful designs. They simply show onegroup of structures that yield useful filters of either additive orsubtractive primary bands.

Filter designs based on FIG. 7 were generated by incrementing throughvalues of α₁ (5, 10, 15, 20 . . . ) and, for each value of α₁,incrementing through values of α₂. The transmission spectrum of eachfilter was calculated and useful spectra were identified. The spectracan be calculated using Mueller or Jones matrices. Preferred spectra fordisplay applications have a duty ratio matching the desired spectrum andhave a rectangular looking profile. Preferably the filter has a steeptransition slope, with a relatively flat transmission band and arelatively flat blocking band. Relatively flat bands can be achieved byhaving a series of distributed high-contrast nulls or peaks in thespectrum. Near ideal transmission spectra can be produced by increasingthe number of retarders. In practice, acceptable transition slopes andside-lobe amplitudes/locations must be judiciously chosen to optimizesaturation with a limited number of components.

The filter resolution must be sufficiently low to sustain peaktransmission throughout the primary band. From a saturation standpoint,the pass-band resolution must be sufficiently high to isolate only thedesired primary band. Designs which produce sufficiently low resolution,along with steep transition slopes (or multiple peaks in the pass-band),are preferred.

Some of the useful designs are shown in Table I. The retarders for bothstacks are listed in the table as they would appear in the normalswitching state, wherein the LCD is not seen. The total number ofretarders, M, is listed. Note that for equal retardances theorientations follow Eq. 1. Suitable designs obtained by stepping throughα₁ and α₂ can be improved by fine tuning of the angles. Afteridentifying the basic design, the retardance Γ can be selected toproduce the desired subtractive or additive primary modulation.

Fan olc filters are a family of structures that happen to conform to thedesign requirements of FIG. 6. This conformation is a fortuitouscoincidence and is in no way a deliberate feature of the olc design. olcfilters are described in U.S. Pat. No. 5,469,279, and U.S. patentapplication Ser. No. 08661498 filed Jun. 11, 1996, which areincorporated by reference herein in its entirety. A olc filter requiresa series of identically thick retarders which are full-wave or half-waveretarders at the design wavelength for fan and folded designs,respectively. For fan olc filters the retarders are oriented at α, 3α,5α, . . . (i.e. α_(N) =(2N-1)α₁). For folded olc filters they areoriented at alternating rocking angles α, -α, α, . . . (i.e. α_(N)=(-1)^(N+1) α₁).

Table II shows fan olc designs according to these requirements. Notethat the fan olc designs coincidentally fit the requirements of FIG. 6.When M is odd, the center retarder in a fan olc design is oriented at45°, at which angle α=90-α. Thus the center retarder is simply split inhalf, and half is included in each retarder stack. The total number ofretarders M is listed in quotation marks as "3" or "5" for cases where,in the normal state, the center two retarders are equivalent to a singleretarder. The retarder orientation is a function of the total number ofretarders, where α=π/4M.

In the case of the fan olc filter, the filter naturally fits thecriteria of the white/primary filter of the present invention. Thefolded olc filter, on the other hand, does not fit the present criteria.The retardances meet the constraints but the orientations do not obeyα_(N) =90±α_(N). The folded olc design can be modified into aquasi-folded olc design which does meet the criteria, as shown in TableIII. The term quasi-folded olc design is used for all designs whereinthe retarders all have the same retardance and wherein, within eachstack, they are all oriented at approximately the same angle but withalternating sign.

In a classical folded olc filter, as in the fan filter, α=π/4M. Forexample, the classical folded olc filter with N=6 retarders calls forα=8°. For the quasi-folded olc filter there are no limitations on theangle α. As shown in Table III it can be varied substantially whilestill producing useful spectral responses. Tables I-III show NF designs.To make NW designs the sign of the angle for each retarder in the secondstack is reversed.

Like the olc filter, the split-element filter naturally fits therequirements of FIGS. 5 and 6. A split-element filter suitable for colordisplay is described in U.S. Pat. No. 5,528,393, which is incorporatedby reference herein in its entirety. It comprises first and secondmatched retardance split-element retarders oriented at ±45° with respectto the entrance polarization, and a center retarder oriented at 0°. Forparallel split-elements the filter is NF, and for crossed split-elementsit is NW.

To form a white/primary filter of this invention, the split-elementretarders form the stacks and the center retarder can be positionedbetween the LCD and one of the split-element retarders. In thisasymmetric case the retarders on either side of the LCD are not matched,but the stacks can be considered to each comprise a single retarder, thesplit-element retarder, and the center retarder can be considered anaddition element not included in either stack. Because the centerretarder is oriented parallel or perpendicular to the electro-opticmodulator the lack of symmetry does not prevent the attainment of awhite switching state. The referred embodiment is shown in Table IV. Inthe table the center retarder is listed along with the Stack 1 retarder.For the retardances shown, the filter transmission approximates atwo-stage Lyot filter. Because of the quarter-wave retardance added toeach split-element retarder, the design belongs to the set of compleximpulse response filters.

For all the designs described above, once the orientations have beenselected for the optimum profile, the retardances can then be selectedto provide optimum color saturation at each primary. The designparameters can be analyzed with standard Mueller matrix techniques,which include a dispersion fit to specific retarder materials. Thecriteria for evaluating filter designs is based on considerations ofsaturation, hue, and throughput. The saturation and hue can be evaluatedusing the CIE chromaticity diagram. The quality of color generated by aparticular filter output can be characterized by calculating a series ofoverlap integrals, including the transmission function for a specificfilter state, the power spectrum of the source, and the CIE colormatching functions.

Saturated primary colors are generated by maximizing the ratio betweensource power transmitted in the desired primary band to that transmittedoutside of the primary band. The filter design can be matched to thesource characteristics to make optimization quite specific. For example,true white sources, such as a 6000 K black body, place greater demand onfilter performance than distributed sources, such as a CRT phosphor. Thespectral positions of nulls in the blocking band depends upon theretardance of the components. It is advantageous to strategically placenulls at out-of-band power spectral maxima of the light source.Similarly, it is advantageous to place side-lobe maxima away fromout-of-band power spectral maxima. Passive filters can be inserted toreject bands that lie outside of the primary color bands to increasesaturation.

Measured spectra of white/primary filters are shown in FIGS. 8-11. Thestructures of the filters are given in Table V. The switches use a 3TNcell with an unenergized half-wave retardance center wavelength aslisted in the table. The polarizers are Nitto-Denko EG1425 with only ahard-coat. The transmission therefore includes two Fresnel losses. Theretarders are sheets of NRZ retarder, each having the design retardanceslisted in the table. For the NW designs the losses in the white stateare associated with the polarizers, absorption by ITO electrodes on theLCD, external reflections, and any LC residual retardance. The latter istypically about 20 nm. The spectra were scanned using an ANDO opticalspectrum analyzer, automatically normalized by the source spectrum withparallel Glan-Thompson polarizers.

The design wavelengths of the passive and LCD retarders are given inTable V. For both active and passive retarders, the design wavelength isthe wavelength at which the retarders give the specified retardance. Forthe passive retarders it is the wavelength at which they are full-waveretarders, except for the W/M filter in which they are 2λ retarders. Theretarder stack design wavelength is chosen to place the modulatedblocking peak in the desired color band. The design wavelength of theLCD is the wavelength at which the retardance is λ/2 in the unenergizedstate. Note that, in order to minimize the effect of LCD chromaticity,the design wavelength is chosen to fall in the center of the modulatedband.

The G/W, W/C and W/M filters are quasi-folded olc designs. The G/Wdesign is the same as the third design in Table III. The W/M design isthe same as the second design in Table III, except that since it isnormally white rather than normally filtered the orientations of theretarders in the second stack have reversed signs. In the W/C design theorientations of the retarders have been adjusted to increase steepnessof the transition. The W/Y design can be recognized as the fifth designin Table I, where retardance Γ=2λ.

Note the excellent passband transmission, stopband blocking and steeptransition edge of the filter spectra of FIGS. 8-11. The reduced filtertransmission in the blue is due to the use of sheet polarizers which arehave losses in the blue, and not to the retarders or filter design.Transmission can be improved with the use of improved polarizers.

A benefit of this invention is that the LCD has no mechanism forinducing color shift. That is, the transition bandwidths are defined bythe retarder stacks independent of the state of the LCD. As such,changes in view angle (or LCD chromaticity) have very little effect onfilter transmission. Features such as transition band centerwavelengths, defined by the stacks, are preserved to the extent that thestacks are angle independent. Changes in view angle produce only aslight loss in density of the blocked color and no shift in bandposition.

This is demonstrated with the W/M filter of FIG. 10. For an azimuthangle of 0°, the measured change in transmission with angle of incidenceis shown in FIG. 12. For incidence angle ranging as far as -50° fromnormal, the spectrum is remarkably unchanged. The magenta transmissiondecreases slightly but the green blocking remains excellent and the bandpositions are unchanged. The half-max of the blue/green and red/greentransitions is fixed by the stack. The spectrum varies with azimuthalangle as well as with incidence angle. The worst azimuthal anglemeasured was 90°, as shown in FIG. 13. The 0° incident angletransmission is actually identical for either azimuthal angle, and themeasured difference between FIGS. 12 and 13 is an artifact of thepolarization-dependent coupling of the light source via an opticalfiber. Even in the worst case of 90°. azimuth angle, the blockingdensity is excellent and the bands are not shifted. This makes forexcellent wide view-angle color switches, and display systems.

A unique feature of subtractive displays based on this invention is theability to use an analog LCD for gray level control of the transmissionof the modulated light without affecting the transmission of theunmodulated primary. This is illustrated in FIG. 14, which is theexperimentally measured output of the W/M filter of FIG. 10.Transmission spectra are shown at different drive voltages. The filteris normally white and therefore fully transmits all wavelengths in thezero retardance state (10 V). As the retardance increases, thetransmission of the modulated primary, green, is gradually blocked untilit is minimized in the half-wave retardance state (0V). The filterdemonstrates independent modulation of green light without affecting themagenta light.

Gray scale modulation is further demonstrated in FIGS. 15-17 for W/C,W/M and W/Y filters, respectively, wherein the transmission of theadditive primaries R, G and B is controlled with complete independencefrom the complementary subtractive primaries. The designs of thesefilters are given in the first three entries in Table VI. The retardersare Nitto Denko polycarbonate films. The optical modulator is azero-twist nematic oriented at 0°, and continuously variable betweenzero and half-wave (π) retardance. The out of plane tilt angle of thenematic is labeled in the spectra, with 0° corresponding to half-waveretardance and 75° corresponding to approximately zero retardance. While90° would be more nearly zero retardance, it is difficult to achievethis tilt due to surface pinning effects. A passive retarder can be usedto compensate for residual retardance.

A M/W filter is shown in FIG. 18 and listed at the bottom of Table VI.It is a split-element filter with the center retarder included with thefirst stack. In this filter the LCD is oriented at 90° rather than 0°.For the other filters, the filter function is identical for retarderorientations of 0 and 90°. For the split-element design, because of theasymmetry the spectra are not identical, although both orientations arefunctional. For this particular filter, evaluation of the twoorientations indicated that 90° orientation provides a better output.The filter can be varied continuously between the two extreme switchingstates shown in FIG. 18. Because this filter is NF, the white spectrumsuffers from chromaticity.

A unique characteristic of the subtractive filters of this invention, asshown in FIGS. 15-17, is that the unmodulated spectrum F is fullytransmitted, independent of the voltage-controlled modulation of the Fspectrum. As a result, two or more filter stages can be used in series,each stage providing independent analog control of one additive primarywithout affecting the other two. A one stage filter has two outputs, anadditive or subtractive primary and either black (crossed polarizers) orwhite parallel polarizers). A two stage filter can provide four outputs,three primary colors (two additive and one subtractive or one additiveand two subtractive) and either black or white. A three stage filter canprovide eight outputs, three additive primaries, three subtractiveprimaries, black and white. If the modulators are analog, the filterscan additionally provide gray scales between the color extremes. In amultiple stage filter the chromaticity can be reduced as described inU.S. patent application Ser. No. 08/758,122, filed Nov. 25, 1996, whichis incorporated by reference herein in its entirety. A multiple stagefilter is also described in U.S. patent application Ser. No. 08/645,580,filed May 14, 1996, which is also incorporated by reference herein inits entirety.

The filters can be combined utilizing an entrance and exit polarizer ineach stage, as shown in FIG. 19. In this case, the filter output is theproduct of the outputs of each individual stage. The first stagecomprises retarder stacks 20a and 30a, LCD 10a, and polarizers 40a and50a. There are n stages with polarizers between each stage, ending withthe nth stage comprising retarder stacks 20n and 30n, LCD 10n, andpolarizers 40n and 50n. The output polarizer of each stage serves as theinput polarizer for the next. For example, if there were only two stages50a and 40n would be the same polarizer.

By proper selection of the filter stages and their relative orientationsit is possible to combine two or more stages without the need forinternal polarizers between the stages, as shown in FIG. 20. Sincepolarizers can be a major source of light loss, the multiple stagefilters without internal polarizers can have significantly increasedthroughput, particularlly for reflection-mode color switches anddisplays.

The three stage filter of FIG. 20 has stages which independentlymodulate blue, green and red light, placed between polarizers 40 and 50.In this embodiment, the first stage modulates blue light and istherefore either a W/Y or Y/W filter stage. The stage comprises firstretarder stack 20a, second retarder stack 30a, and LCD 10a. The retarderstacks and LCD can be any of the designs of this invention. The secondstage, comprising retarder stacks 20b and 30b and LCD 10b, modulatesgreen light and is therefore a W/M or M/W filter. The third stage,comprising retarder stacks 20c and 30c and LCD 10c, modulates red lightand is therefore a W/C or C/W filter.

A specific three-stage filter is illustrated in FIG. 21. These are thesame filter stages as FIGS. 9-11, but combined in series withoutinternal polarizers. The output colors are given in Table VII. For eachstage a zero refers to the unenergized (modulating) LCD state, and a onerefers to the energized (isotropic) state. When all three stages are intheir modulating states, the first blocks blue, the second blocks greenand the third blocks red, resulting in a black output when used withparallel input and output polarizers. When the third stage LCD isswitched to the isotropic state, it no longer blocks red and the filteroutput is red. When all three LCDs are switched to their isotropicstates, the output is white.

The white output can be three times as bright as either a spatiallymultiplexed filter or display, wherein subpixels of red, green and bluecombine to make white, or a temporally multiplexed filter or display,wherein the output of the pixel switches between red, green and blue tomake white. In the filter of this invention, the full white spectrum canbe transmitted over the entire space and time.

In general, if the polarizers are crossed instead of parallel, thecomplementary spectrum is obtained. For example, a cyan/white filterwith parallel polarizers is instead a red/black filter with crossedpolarizers. For the case where three stages are cascaded withoutinternal polarizers, a white state can be achieved even with crossedpolarizers. Because no colors are blocked by internal polarizers, thefull complementary spectrum is available on the orthogonal axis. Thecrossed polarizer outputs are included in Table VII. The state (000)gives white instead of black, (001) gives cyan instead of red, and soon.

An advantage of a crossed polarizer filter is the improved opticaldensity of the black state. The black state is output when all threeLCDs are in their energized, isotropic, switching states and thereforehave minimal chromaticity. It is generally preferable to suffer someloss of throughput in the white state in exchange for increased blockingin the dark state.

For the filter of FIG. 21 having crossed polarizers, the additiveprimary outputs are shown in FIG. 22 and the subtractive primaries inFIG. 23. Excellent spectra for all six primaries are provided by asingle filter, with analog control of each primary available. Inaddition, white, black and gray outputs are provided, as shown in FIG.24. Tilt angles from 0 to 75° are illustrated, corresponding toretardances of λ/2 through slightly greater than zero. Note theexcellent black state achieved. To achieve gray scale modulation overthe entire visible spectrum, as shown in FIG. 24, the tilt angles of allthree LCDs are varied simultaneously.

Eliminating internal polarizers, can improve the black state withparallel polarizers, by allowing the stacks to cooperate in blocking theinter-primary bands. Alternatively there is less ripple on the whitestate with crossed polarizers. To see this, consider a subtractivedisplay or color shutter consisting of a stack of C/W, M/W, and Y/Wswitches. Arbitrarily, take the transition band of the yellow andmagenta stages, and that of the magenta and cyan stages to overlap atthe half-maximum transmission points. The individual additive primaryspectra are illustrated in FIG. 25a.

Initially, consider the case with parallel neutral polarizers betweeneach switch, as shown in FIG. 19. Since neutral polarizers separate thestages, the dark state is the product of the C, M and Y spectra producedby each stage. Since the spectra overlap at the half-maximum, theleakage at the center of the transition band is 25%, as shown in FIG.25b. This, depending on the characteristics of the source spectrum, canrepresent a significant loss in density of the dark state. It alsorepresents a blue/green side-lobe of the red output and a yellow sidelobe of the blue output.

There are solutions that reduce the level of these inter-primaryleakages. For instance, the region of overlap can be reduced by shiftingthe color polarizer spectra further apart. In order to ensure that, forinstance, green is fully passed by the yellow stage and blue is fullypassed by the magenta stage, this often means that the transition slopemust be increased. This represents additional retarders, and theassociated cost. Alternatively, light in the inter-primary bands can inprinciple be removed using passive notch filters. This also representsadditional filtering, along with the associated insertion loss and cost.An even better solution, but not often viable, is to use a source thatsimply emits no light in the inter-primary bands.

Another approach to the problem is to remove the polarizers between thestages, as illustrated in FIG. 20. An additional payoff is theelimination of the loss associated with two neutral polarizers. Thechallenge is to identify a scheme whereby the stacks work together toimprove the rejection in the inter-primary bands, rather than tocompound the problem. The solution can be illustrated by considering thestate of polarization of light at the half-max overlap wavelengthsbetween yellow and magenta or magenta and cyan stages. Light exitingeither stack is in general polarized intermediate between the orthogonallinear states at the overlap wavelengths. This represents the set ofpolarization states that have arbitrary ellipticity, with polarizationellipse orientation of ±45 degrees. Provided that the two stacks arecompatible and are properly oriented, the polarization transformationsfrom the two stacks can be cumulative, thereby orthogonally polarizingthe half-max overlap wavelength. For instance, if two stacks function ascircular polarizers at the half-max overlap wavelength, a combinedretardance of half-wave can be achieved. This produces the desired nullin the transition band. FIG. 25c shows that the result is an improvementin optical density over the output of FIG. 25b using additionalpolarizers. Any of these filters can also be combined in series with ashutter to provide a good dark state.

As described above, in the multiple-stage filter without internalpolarizers the full complementary spectrum is available, enabling theuse of crossed polarizers. This feature also allows polarizationdiversity filtering, as shown in FIG. 26. In polarization diversityfiltering the input and exit polarizers are replaced by polarizationseparators and combiners. Unpolarized white light is incident onpolarizing beam splitter 41. One linear polarization is transmitted andthe orthogonal polarization is reflected up to prism 42. The twoorthogonal polarization states propagate independently through thestructure and are recombined by exit prism 52 and polarizing beamsplitter 51. For light in both beam paths, the modulated spectrum exitsin one direction, for which the input and exit polarizations areparallel, and the complementary crossed polarizer spectrum F exits inanother direction. In the polarization insensitive filter, rather thanlosing half the incident light due to absorption by an entrancepolarizer, all of the incident unpolarized light is filtered. Thiseliminates the 3 db loss in efficiency generally associated withpolarization based systems. The filter having polarizing beamsplittersit is particularly suited to small aperture applications. Largeapertures can be achieved with polarizing films that give polarizationdiversity.

Multiple stage filters have been demonstrated using filter stages inseries. In addition, it is feasible to modulate between two independentspectra by nesting retarder stacks within a single stage, as shown inFIG. 27. This is done by orienting one stack to be NW and the other tobe NF. Stacks 20a and 30a in combination with LCD 10 form a NW filterwhich modulates spectrum F₁, thereby transmitting white when the LCD isenergized (anisotropic) and F₁ when it is unenergized. Stacks 20b and30b in combination with LCD 10 form a NF filter which modulates F₂,thereby transmitting F₂ when the LCD is energized and white when it isunenergized. The nested stacks therefore transmit F₂ when the LCD isenergized and F₁ when it is unenergized. Such structures can be stackedwith or without intervening polarizers, and combined with any otherpassive or active filtering.

Reflection mode switches can be implemented by simply following anytransmission mode structure with a mirror. Alternatively, structures canbe designed specifically for reflection mode operation. In addition tothe design rules described previously for white/primary switches,reflection mode filters have additional symmetry requirements. Considera design in which a single stack precedes an LCD, followed by a mirror.The unfolded version consists of two stacks, in which both stackeffectively have the same orientation. A solution involving neutralpolarization optics can be used to create an effectively differentorientation of the second stack. For instance, an achromaticquarter-wave plate can be placed on the mirror to make the second passthrough the stack to appear at a different orientation. However, thischanges the action of the LCD, as the second pass through the LCD alsoappears to have a different orientation. Also, the additional half-waveof retardance inverts the spectrum, making subtractive-primary switchesadditive-primary switches and vice-versa.

If the LCD follows the achromatic quarter-wave plate, then the desiredcondition of doubling the action of the LCD is achieved. However, thequarter-wave plate transforms the state of polarization on the LCD, sothat the combined stack may not provide isotropic/modulation states tothe modulator. In order to solve this problem, either a differentmodulator can be used that is more compatible, or a different stackdesign can be employed. A solution involving the former is to use arotative element, such as a CSLC quarter-wave retarder on the mirror.The overall modulator thus has the structure of the quarter-half-quartervariable retarder as described in U.S. Pat. No. 5,381,253, which isincorporated by reference herein in its entirety. If the modulator mustremain a zero-twist nematic, then the stack design must be modified inorder to provide appropriate isotropic/modulation states to themodulator when combined with an achromatic quarter-wave plate.

For the case where the analyzing polarizer is a CLC, a multiple stagefilter can be designed for both transmission and reflection mode, asshown in FIG. 28. The first stage modulates blue light and comprisesstack 20a, electro-optic modulator 10a and blue CLC 90a. The secondstage modulates green light and comprises stack 20b, electro-opticmodulator 10b and green CLC 90a. The third stage modulates red light andcomprises electro-optic modulator 10c and red CLC 90c. Because the redCLC is isotropic to blue and green light, by orienting electro-opticmodulator 10c so that the red light is in the modulation state, thefinal stage does not require a retarder stack. In the first stage,depending on the variable retardance Γ_(v), a percentage of the bluelight, labeled % B, is transmitted and the remainder is reflected. Thereflected light is indicated by the arrows below the device structure.Because only the blue CLC selectively reflects blue light, i.e. thegreen and red CLCs are isotropic to blue light, the polarization of theblue light is irrelevant after this stage. The second and third stageslikewise reflect and transmit portions of the green and red light.

The filters of this invention can be used in combination with any otheractive or passive filter. Hybrid filters can also be made with activeand passive filters. For example, instead of a neutral polarizer thefilter can employ a color polarizer, such as a dye type color polarizeror a polarizer retarder stack (PRS) color polarizer. In this case the"white" state contains only the wavelengths passed by the colorpolarizer. The white/primary filter can also be combined with apolarization interference filter.

The filters of this invention can be optically addressed. For example,the optical addressing system can include a photodetector such as a PNdiode or a phototransistor which detects an optical signal, and cancontrol the filter output in response to the optical signal.Applications of the optically addressed filter include eye protection,welding shields and color shutter glasses for 2D and 3D display of data.

The filters can be used as a single pixel or in a multi pixel array.Single pixel applications include field sequential color shutters,spectrometry, colorimetry, lighting (home, house, stage) spectroscopyand fiber optic communication. Multiple pixel applications includeinformation display, imaging, printing, analysis and storage andcommunication. In multiple pixel arrays, each pixel can be controlledindependently via an independent applied voltage. Each pixel can provideanalog intensity control of all three additive or subtractive primariessimultaneously, thereby providing the full color spectrum, includingblack and white.

For compatibility with existing devices, the pixels can have subpixelsfor each color, for example W/R, W/G and W/B pixels. The subpixels canbe patterned, for example, in stripes or or square patterns such as theBayer Mosaic or other color filter array patterns (CFAs). Each pixel ofthis invention comprised of subpixels has the advantage over priorspatial multiplexed filters that each subpixel can transmit the entirewhite spectrum rather than only one third of it, thereby increasing thewhite brightness by a factor of three.

Display applications include front and rear projection displays, virtualdisplays and direct view displays. Displays can be used in a variety ofapplications, such as head-up displays in transportation vehiclesincluding automobiles, trucks and airplanes, boardroom projectors,desktop computing, home theater, stage lighting, handheld games, arcadegames (3D and 2D), laptop displays, handheld pagers, personal displayassistants, global positioning displays, instrumentation such asoscilloscopes and spectrum analyzers, web browsers, telecommunicators,head-mounted displays and displays brought to the eye for virtualreality, augmented reality, portable wearable computers, simulators,camcorders and display glasses, goggles or shutters.

For display applications, the multi pixel filter can be used incombination with emissive displays such as cathode ray tubes (CRT),electroluminescent (EL) displays, active matrix electroluminescent(AMEL) displays, field emission displays (FED) and plasma displays. Theycan also be used with modulator displays including transmissive displayssuch as TFT-LCD and polysilicon LCD, reflective displays such as liquidcrystal on silicon (LCOS), digital mirror devices (DMDs) and diffractivegrating devices, and passive matrix displays such as STN devices.

Electronic imaging applications include pagefed and document scanners,internet cameras and document scanners, digital cameras for studiophotography, microscopy, multispectral imaging, documentation such asphoto-ID cameras, amateur electronic photography, and other applicationsincluding fluorescence spectrometry, colorimetry, and medical imagingused with for example endoscopes and other medical diagnostic equipment.

To form imaging devices, the filter of this invention can be combinedwith still or video cameras using Charged Coupled Devices, ChargeIntegrating Devices or Complementary Metal Oxide Semiconductor singlepixel or multi-pixel imagers.

FIG. 29 shows a stack 290 which includes a first retarder 291 and asecond retarder 292 having a first orientation α₁ and a secondorientation α₂, respectively, as well as a first retardance Γ₁ and asecond retardance Γ₂, respectively. FIG. 29 shows partially polarizedlight 294 input to stack 290. Partially polarized light 294 can be inany frequency spectrum of electromagnetic radiation and can be partiallyelliptically polarized with any ellipticity, orientation, or handedness.Partially polarized light consists of a polarized and an unpolarizedcomponent. The unpolarized component is passed unchanged. In theFigures, we refer to the action of the elements acting on the polarizedcomponent of the light. The polarized component can have anypolarization, including any orientation ellipticity, and handedness.See, for example, Chapter 1 of "Optical Waves in Layered Media"Copyright 1988, John Wiley & Sons, New York, incorporated by referenceherein. By partially polarized light it is meant light which is notcompletely unpolarized.

Stack 290 transforms partially polarized light 294 in a known mannerdepending on the values of first and second orientations α₁ and α₂ aswell as first and second retardances Γ₁ and Γ₂. First and secondorientations α₁ and α₂ are measured with respect to the polarization ofpartially polarized light 294. If partially polarized light 294 iselliptically polarized, then α₁ and α₂ can be defined either the axis ofthe input or output polarization ellipse.

Stack 290 transforms partially polarized light 294 into secondpolarization transformed light 296. In particular, partially polarizedlight 294 is received by first retarder 291 and is transformed intoinitially transformed light (not shown). The initially transformed lightoutput from first retarder 291 is then input to second retarder 292.Second polarization transformed light 296 includes a first portion oflight 297 and a second portion of light 298. First portion of light is297 has a first polarization P_(M1) and second portion of light 298 hasa second polarization P_(M2). Also, first portion of light 297 has afirst spectrum F' and second portion of light 298 has a second spectrumF'.

Stack 290 is a light preconditioning device for preconditioning light asan input to some type of light modulation device (not shown). In aspecial case, first spectrum F' and second spectrum F' can becomplements of each other and together comprise the spectrum ofpartially polarized light 294, and accordingly correspond to spectrum Fand complementary spectrum F of FIG. 1. First spectrum F' has firstpolarization P_(M1), which is modulated more than second spectrum F'with second polarization P_(M2). Once partially polarized light 294 isprovided, and the modulator (not shown) characteristics are known, firstorientation α₁ and first retardance Γ₁, as well as second orientation α₂second retardance Γ₂ can be determined in accordance with the abovediscussion. Additional retarders can be added to stack 290 to achievespecified performances as discussed above.

In a special case, first output light 297 has a polarization P_(M1)which is affected or modulated by the modulator, whereas second outputlight 298 with polarization P_(M2) is not modulated by the modulator asdiscussed, for example, with reference to FIG. 1 and elsewhere.Throughout the discussion that follows, the term orthogonal whenreferenced to states of polarization does not necessarily refer to twolinear states that are perpendicular to each other, but instead refersgenerally to any two polarization states including ellipticalpolarization states which have the following property. If a firstpolarizer has a first eigenstate corresponding to the first polarizationand a second polarizer has an eigenstate corresponding to the secondpolarization and if unpolarized light is input to the first polarizerand the second polarizer, no light is output from the second polarizer.That is, the first and second polarizers are "crossed" in a generalsense. Hence, in the special case of linearly polarized light, the firstpolarization and the second polarization are orthogonal when they areperpendicular. In the special case of circularly polarized light, thefirst polarization and the second polarization are orthogonal when thehandedness of the first polarization is opposite the handedness of thesecond polarization, i.e., the first polarization could be clockwise andthe second polarization could be counter clockwise, or vice versa.Finally, in the case of elliptic polarization (of which circularpolarization is a special case), the first polarization would have amajor and minor axes that are respectively perpendicular to the majorand minor axes of the second polarization.

Referring back to FIG. 29, first polarization P_(M1) and secondpolarization P_(M2) cannot be equal and may or may not be orthogonal.Partially polarized light 294 may or may not be visible light. Asdiscussed above, stack 290 and in particular, first retarder 291 andsecond retarder 292 can be polymer retarders, liquid crystal polymerretarders, form birefringent material, polymer birefringent retarders,liquid crystal polymer retarders, birefringent-crystals, liquidcrystals, etc. Stack 290 can and does include all the possible materialsand retarders discussed above with respect to stack 20 of FIG. 1. Hence,first orientation α₁ and second orientation α₂ are not equal, but firstretardances Γ₁ and second retardance Γ₂ may or may not be equaldepending on the polarization of partially polarized light 294 and thedesired polarization P_(M1) and P_(M2) of first output light 297 andsecond output light 298, respectively.

FIG. 30a and 30b show two general examples of partially polarized light294 and 294' input to stacks 300 and 300' corresponding stack 290 ofFIG. 29. Here, second polarization transformed light 296 includes firstspectrum F' having polarization P_(M1) ' and second spectrum F' havingpolarization P_(M2) wherein P_(M1) and P_(M2) are linear polarizationsand are not necessarily perpendicular. Second polarization transformedlight 296' includes first spectrum F" having polarization P_(M1) andsecond spectrum F" having polarization P_(M2) where first polarizationP_(M1) second polarization P_(M2) are not necessarily orthogonal.

FIGS. 31a and 31b show stacks 300 and 300' with the addition ofpolarizers 311 and 311', respectively. FIG. 31a corresponds to thesituation in which second transformed output light 296 includes a firstportion 297 and a second portion 298 which are linearly polarized withfirst polarization P_(M1) and second polarization P_(M2). FIG. 31bcorresponds to the case in which the second transformed output lightincludes a first portion of light 297 and a second portion of light 298which are elliptically polarized with polarizations first polarizationP_(M1) and second polarization P_(M2), respectively. In both cases,unpolarized light 312 is polarized at least partially by polarizers 311,and 311' to yield partially polarized light 294 and 294' correspondingto FIGS. 30a and 30b. Polarizers 311 and 311' can be the same aspolarizer 40 in FIG. 1 again, polarizers 311 and 311' can includeabsorptive type polarizers such as dichroics, or dye-based polarizers ornon-absorptive including cholesteric LC's, cholesteric polymer LC's,retarder-based polarizers and splitters. Polarizers 311 and 311' canalso include non-absorptive type polarizers such as polarizingdielectric films including those used in polarizing beamsplitters,calcite, quartz, scattering polarizers, prismatic polarizer, stackedcholesteric with lambda/4 plates, or other cholesteric type polarizers.

FIGS. 32a and 32b show a device 321 for manipulating partially polarizedlight 294 and 294' and outputting resulting modulator output light 325and 325', respectively. Referring to FIG. 32a, manipulating device 321includes retarder 322 and modulator 320. Retarder 322 is similar toabove discussed retarder 291 and has an orientation α₁ with respect topartially polarized light 294 has retardance Γ₁. Partially polarizedlight 294 passes through retarder 322 and is transformed to transformedlight 296 having a first portion 297 with first polarization P_(M1) anda first spectrum F' as well as a second portion 298 having a secondpolarization P_(M2) and a second spectrum F'. Transformed light 296 isthen input to modulator 320 which modulates first portion 297 in adifferent way than it modulates second portion 298. In a special case,second polarization P_(M2) can be selected to correspond to a state inwhich modulator 320 appears isotropic to second portion 298 regardlessof the voltage driving modulator 320. In that case, which corresponds tothe above discussion, second polarization P_(M2) remains unchanged orunaffected as transformed light 296 passes through modulator 320,regardless of the state of modulator 320 (e.g., regardless of thedriving voltage applied to modulator 320). Here, if second portion 298is in such an isotropic state, then first portion 297 must not be insuch an isotropic state (e.g., it must be modulated by an applieddriving voltage).

In an alternative embodiment, first portion 297 with first polarizationP_(M1) should be modulated by modulator 320 in a manner different fromsecond portion 298 with polarization P_(M2). As can be seen in thefigure, first portion 297 with polarization P_(M1) is received andmodulated by modulator 320 to yield a first portion 323 of modulatoroutput light 325 having a third spectrum F₃ and a third polarization P₃.Also, second portion 298 having second polarization P_(M2) and P_(M2)second spectrum F' is received by modulator 320 which outputs secondportion 324 of modulator output light 325 having a fourth spectrum F₄and a fourth polarization P₄. Generally speaking, the manner in whichfirst portion 297 is affected by modulator 320 is not the same as themanner in which second portion 298 is affected by modulator 320 for allvoltages (there may be certain voltages in which both are affected inthe same manner).

FIG. 32b corresponds to FIG. 32a but with elliptically polarized lightinstead of linearly polarized light input to modulator 320. Inparticular, transformed light 296' includes first portion 297' with afirst polarization P_(M1) ' and a first spectrum F", and a secondportion 298' with a second polarization P_(M2) ' and a second spectrumF'. First portion 297' of transformed light 296' is received bymodulator 320 and transformed to a third portion 323' of modulatoroutput light 325' having a third polarization P₃ ' and a third spectrumF₃ '. Second portion 298' of transformed light 296' is transformed bymodulator 320 to yield a second portion 324' of modulator output light325' having a fourth polarization P₄ ' and a fourth spectrum F₄ '. In aspecial case, polarization P_(M2) ' is in an isotropic state ofmodulator 320 such that second portion 298' of transformed light 296' isnot affected or not modulated by modulator 320. In that case, firstportion 297' of transformed light 296' is modulated by modulator 320regardless of any driving voltage applied to modulator 320.

Modulator 320 can be an electro-optic modulator, a magneto-opticmodulator or any other optical unit that can modulate light. Hence,modulator 320 can include a liquid crystal such as a nematic liquidcrystal, surface mode device, a twisted nematic, a super twist nematic,an electrically controlled birefringent, optically controlledbirefringent, a hybrid field effect, and hybrid aligned nematic device.Alternatively, modulator 320 can include a pi-cell, a zero twist modecell, guest-host dye liquid crystal device. Also, modulator 320 caninclude a smectic liquid crystal, a chiral smectic material, and caninclude FmC*, a surface stabilized FLC, a volume stabilized FLC, aFmA*-electroclinic, a distorted helix ferroelectric, ananti-ferroelectric, a flexoelectric, and an achiral ferroelectric liquidcrystal. Retarder 322 can be made of any of the materials used for theretarders in the above discussed stacks.

FIG. 33a shows a device 330 for manipulating at least partiallypolarized light 294. In particular, device 330 includes retarder 322 andmodulator 320 for FIGS. 32a and 32b. Device 330 further includes aretarder 331, wherein retarder 322 is on one side of modulator 320 andretarder 331 on the opposite side of modulator 320. Referring to FIG.33a, partially polarized light 294 transmits through retarder 322 andmodulator 320 which outputs intermediate light 326 including firstportion 323 and a second portion 324. First portion 323 of intermediatelight 326 has a first spectrum of F₃ and a third polarization P₃, and asecond portion 324 of intermediate light 326 as a fourth spectrum F₄ anda fourth polarization P₄ as discussed above with reference to FIG. 32a.First portion 323 of intermediate light 326 transmits through secondretarder 331 which transforms first portion 323 of intermediate light326 into 333 of output light 335. First portion 333 of output light 335has a fifth spectrum F₅ and a sixth polarization P₅. Also, secondportion 324 of intermediate light 326 passes through second retarder 331which transforms it into second portion 334 of output light 335 having asixth spectrum F₆ and a sixth polarization P₆. First retarder 322 has afirst orientation α₁ and a first retardant Γ₁ and a second retarder 331has a second orientation α₂ and a second retardant Γ₂, wherein firstretardant α₁ and a second retardant α₂ are measured with respect to apolarization direction of partially polarized light 294. As before, themanner in which modulator 320 modulates first portion 297 should not bethe same as the manner in which modulator 320 modulates second portion298.

In one particular embodiment, first orientation α₁ and first retardantΓ₁ can be selected such that first portion 297 of transformed light 296is modulated by modulator 320 and second portion 298 of transformedlight 296 is not modulated by modulator 320, i.e., modulator 320 appearsisotropic to second portion 298. In this case, second portion 324 ofintermediate light 325 is not changed or modulated or affected bymodulator 320. Hence, F' is approximately the same as F₄ and P_(M2) isapproximately the same as P₄.

In another particular embodiment, secondary orientation α₂ and secondretardance Γ₂ can be selected such that fifth polarization P₅ isperpendicular to sixth polarization P₆ and fifth spectrum F₅ iscomplementary to sixth spectrum F₆. In such a situation, an analyzercould be used to electively filter out either first portion 333 orsecond portion 334 of output light 335, to yield fifth spectrum F₅ orsixth spectrum F₆, respectively.

In another embodiment, first orientation α₁ and first retardance Γ₁ canbe selected such that modulator 320 has at least one state in whichfirst portion 297 is modulated and second portion 298 is not modulated.

In another embodiment, first orientation α₁ can be at an angle α andsecond orientation α₂ can be at an orientation of 90±α.

In yet another embodiment, second spectrum F' of second portion 298transmits to second portion 324 in which fourth spectrum F₄ isessentially the same as second spectrum F' and fourth polarization P₄ isessentially the same as second polarization P_(M2) for all drivingvoltages while first portion 297 of transformed light 296 iscontinuously varied in accordance with the driving voltage of modulator320 so as to yield a continuously varied first portion 323 ofintermediate light 325. A special case of this would be that secondorientation α₂ and second retardance Γ₂ would be selected such thatfirst portion 333 of output light 335 would vary from 0 to fifthspectrum F₅ and fifth polarization P₅ in a continuous manner as thedriving voltage across modulator 320 is continuously varied.

FIG. 33b corresponds to FIG. 33a for the case in which partiallypolarized light 294' is elliptically polarized.

In a preferred embodiment, modulator 320 is switchable between a firstand a second switching state wherein the first switching statecorresponds to a modulation state of polarization and the secondswitching state corresponds to an isotropic state of polarization. Inthis case, first orientation a, and retardance Γ₁ for retarder 322 areselected such that first spectrum F of first portion 297 of transformedlight 296 has first polarization P_(M1) corresponding to the modulationstate of polarization of modulator 320. Also, first orientation a, andfirst retardance Γ₁ are selected such that second spectrum F' is equalto the complementary spectrum F of first spectrum F and secondpolarization P_(M2) corresponds to the isotropic state of polarizationof modulator 320. Furthermore, second orientation α₂ and secondretardant Γ₂ are selected in a manner such that when modulator 320 is inthe first switching state, fifth spectrum F₅ is approximately equal tofirst spectrum F, sixth spectrum F₆ is approximately equal to thecomplementary spectrum F and the fifth polarization P₅ is orthogonal tothe sixth polarization ₆ P . That is, the polarization of first portion333 is perpendicular to the polarization of second portion 334 of outputlight 335.

For angles between 0 and 90°, the filter provides analog control ofintensity for each color. The first stage modulates green with T_(G)=sin² θ₁. The following two stages do not affect the green transmissionbecause both the cyan and the yellow LCPF transmit green at allpolarizations. The second stage modulates red with T_(R) -sin² θ₂, andthe red light is unaffected by the third stage. The problem arises withthe blue modulation. Only third stage polarizer 7 polarizes blue light,but both LCDs 4 and 6 affect the orientation of the blue light.Therefore, TB=sin² (θ₂ +θ₃), and it is not controlled independently ofT_(R).

This problem can be solved with the color selective polarizationmodulator of this invention. FIG. 35 illustrates the polarizationmodulator, comprising stack 20 and modulator 10, for the case where themodulation and isotropic states of the electro-optic modulator arelinear and circular polarizations, respectively. Twisted nematic liquidcrystal cells fall in this class. Retarder stack 20 transforms cyanlight into circularly polarized light, in this example right-handed, andleaves red light linearly polarized. When TN 10 is in the 0 state (FIG.35a), the red light is rotated to 90° and the circularly polarized cyanlight remains circular. In the 1 state (FIG. 35b), the red light remainsat zero and the cyan light remains circular. Electro-optic modulator 10can rotate the red light in an analog fashion between 0 and 90°, but thecyan light always remains circular. The polarization modulated light canbe analyzed with a polarization analyzer utilizing second retarder stack30 in combination with cyan LCPF 80. The retarder stack transforms thecircular light back to linear and leaves the linear light linear. Colorpolarizer 80 transmits the cyan light in all switching states andtransmits a variable intensity of red light.

A key feature of the polarization modulator of FIG. 35 is that itselectively modules red light but, unlike the second stage (elements 3,4 and 5) of the Plummer device, it does not modulate blue light. Theadvantage of this is illustrated in FIG. 36. The first stage usesmagenta LCPF 80a, LCD 10a and neutral polarizer 40 to modulate greenlight. Second stage elements 20, 10b and 30 modulate the polarization ofred light without modulating the polarization of blue light. Since noelements beyond the first stage polarize green light, it does not matterwhether green light falls in the modulation state, the isotropic state,or both, of electro-optic modulator 10b. The blue light, which isunaffected by the second state, is intensity modulated by LCD 10c incombination with yellow LCPF 80c. In this three stage filter, eachprimary color is independently intensity modulated.

Polarizer stack 20 circularly polarizes spectrum F and linearlypolarizes spectrum F. The simplest stack is a single quarter-waveretarder oriented at 45°, with a design wavelength somewhere in spectrumF. Better polarization control can be achieved with a stack whichprovides quarter-wave retardance spectrum F but provides quarter-waveretardance throughout spectrum F but provides full or halfwaveretardance in spectrum F. Such a stack can be called a narrow andachromatic compound quarter-wave retarder. Compound achromatic retardersare described in U.S. patent application Ser. No. 08/491,593, which isincorporated by reference herein in its entirety. An embodiment of anachromatic retarder stack has three retarders of equal retardance, atangles π/12, 5π/12 and π/12. The angles can be varied and the retardanceof the three retarders selected to tailor the wavelength, bandwidth andtransition edge of the quarter-wave retarding spectral region. Retardersat 14°, 85° and 14° are an example of a suitable stack.

The filter of FIG. 36 is but one embodiment of a filter using a TN inthe polarization modulator. It uses retarder stacks in only one stage.Other filters can be designed. For example, the neutral polarizer canfollow rather than precede the color selective polarization modulator.The order in which the colors are filtered can be different, althoughhaving the first stage modulate green reduces the transition edgesharpness required of the retarder stack. The stage with the colorselective polarization modulator can be combined with other colorshutter systems known in the art, not limited to shutter using LCPS.

The filters of this invention can be used in combination with any otheractive or passive filter. Hybrid filters can also be made with active orpassive filter. Hybrid filters can also be made with active and passivefilters. For example, instead of a neutral polarizer the filter canemploy a color polarizer, such as a dye type color polarizer or apolarizer retarder stack (PRS) color polarizer. In this case the "white"state contains only the wavelengths passed by the color polarizer. Thewhite/primary filter can also be combined with a polarizationinterference filter.

Another in-line filter of this invention uses a first stage totemporally multiplex two primary colors with a second stage dedicated tothe third primary, as shown in FIG. 37. This is a hybrid between filterswhich temporally multiplex all three primaries and the subtractivefilters of this invention having one stage for each primary. Colorshutter 100 alternates between transmitting two subtractive primaries,in this example cyan and magenta. The additive primary which is commonto these, blue, is modulated by the second stage and the other additiveprimaries, red and green, are alternately modulated by the first stage.The color shutter can be any shutter which switches between transmittingtwo subtractive primaries. It can contain a liquid crystal cell. Even ifLCDs 10a and 10b are pixelated, the color shutter need not be.

To pass blue light unaltered through the first stage, it is converted tocircular light by retarder stack 20, while red and green (yellow) remainlinearly polarized. Electro-optic modulator 10a rotates the red andgreen light by θ₁, but not blue, and second stack 30 restores the blueto linear polarization at 0°. Second stage modulator 10b rotates theblue light by θ₂ and LCPF 80 transmits a fraction of the blue lightwhich depends on θ₂. Modulator 10b also rotates red and green light, butsince LCPF 80 transmits red and green regardless of polarization, it hasno effect on the red and green output.

An advantage of the two stage filters of FIG. 37 over filters whichtemporally multiplex all three colors is a reduction in the operatingspeed required to prevent perception artifacts. Since one primary isdisplayed all the time, there is reduced flicker. If green is theprimary displayed in each frame there is increased brightness. If it isred, the color balance can be improved. Rather than temporallymultiplexing the first stage using color shutter 100, the first stagecan be spatially multiplexed by using a pixelated color passive colorfilter in combination with pixelated LCD 10a. The uniting feature isthat the second stage is dedicated to one primary, and that primary isunmodulated by the first stage due to retarder stack 20. The final stagecan use any means to modulate the blue light, such as a bluecholesteric, as long as the red and green are not modulated.

The foregoing embodiments are merely exemplary and are not to beconstrued as limiting the present invention. The present teaching can bereadily applied to other types of apparatuses. The description of thepresent invention is intended to be illustrative, and not to limit thescope of the claims. Many alternatives, modifications, and variationswill be apparent to those skilled in the art.

                  TABLE I                                                         ______________________________________                                        Filter Designs                                                                         α,Γ                                                      Retarders M                                                                              Stack 1        Stack 2                                             ______________________________________                                        4          10,Γ                                                                            -20,Γ/2                                                                            -70,Γ2                                                                        80,Γ                                4                 15,Γ                                                                         -45,Γ/2                                                                               75,Γ,Γ/2                     4                 80,Γ                                                                         25,Γ/2                                                                                 10,ΓΓ/2                     4                 60,Γ                                                                        -20,Γ/2                                                                                30,Γ0,Γ/2                    4                 80,Γ                                                                        -25,Γ/2                                                                                10,Γ5,Γ/2                    4                 34,Γ                                                                         11,Γ                                                                                     56,ΓΓ                     4                 45,Γ                                                                        -22,Γ                                                                                    45,Γ8,Γ                    4                 45,Γ/2                                                                     15,Γ                                                                                       45,Γ/2AMMA.                     4                 20,Γ/2                                                                    -10,Γ                                                                                      70,Γ/2Γ                    ______________________________________                                    

                  TABLE II                                                        ______________________________________                                        Fan olc Designs                                                                          α,Γ                                                    Retarders M  Stack 1     Stack 2                                              ______________________________________                                           2         22,Γ  68,Γ                                           "3"                         45,Γ/2 75,Γ                           4                                     56,Γ 79,Γ                   "5"                    9,Γ 27,Γ 45,Γ/2                                                   45,Γ/2 63,Γ 81,Γ               6                              53,Γ 68,Γ 83,Γ               ______________________________________                                    

                  TABLE III                                                       ______________________________________                                        Quasi-Folded olc Designs                                                                 α,Γ                                                    Retarders M  Stack 1     Stack 2                                              ______________________________________                                        4            11,Γ -11,Γ                                                                    -79,Γ 79,Γ                               6                           82,Γ -82,Γ 82,Γ                 6                              75,Γ -75,Γ 75,Γ              6                              22,Γ -22,Γ 22,Γ              ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        Split-Element Design                                                                      α,Γ                                                    Retarders M  Stack 1    Stack 2                                              ______________________________________                                        3             45,Γ + 2  0,Γ                                                                45,Γ + 2                                       ______________________________________                                    

                  TABLE V                                                         ______________________________________                                        Measured Filter Designs                                                       α,Γ           nm)                                                 Color                                                                              Stack 1      LCD     Stack 2   Stacks                                                                              LCD                                 ______________________________________                                        G/W  15,20 45,1   0, 2    45,1 75,2 540   540                                 W/C         12,0 -12,0 12, 0, 2                                                                  -78,0 78,0 -78,      450                                                               650                                               W/M       82,20 -82,20 82,2 0, 2                                                                   -8,20 8,20 -8,2  435                                                                 540                                               W/Y          80,20 -25,      0, 2                                                                    65,0 -10,2       600                                                               430                                               ______________________________________                                    

                  TABLE VI                                                        ______________________________________                                        Specific White/Subtractive-Primary Designs                                    α,Γ           mn)                                                 Color                                                                              Stack 1      LCD     Stack 2   Stacks                                                                              LCD                                 ______________________________________                                        W/C  10,0 -12,0 10,                                                                             0, 2    -80,0 78,0 -80,                                                                         450   660                                 W/M        8,0 -8,0 8,     0, 2                                                                  -82,0 82,0 -82, 760                                                                  540                                                 W/Y         80,20 -25,  0,2                                                                      65, 0 -10, 2  590                                                                    440                                                 M/W        45,0 + 4 0,   90, 2                                                                  45,0 + 4                                                                              540       540                                       ______________________________________                                    

                  TABLE VII                                                       ______________________________________                                        Three Stage Filter Output                                                     Stage                Output                                                   W/Y      W/M    W/C                                                           ______________________________________                                         0            0       0           Black                                                                            White                                    0                        1                    C                               0                        0                    M                               1                        0                    Y                               1                        0                    R                               1                        1                    G                               0                        1                    B                               1                        1               Black                                ______________________________________                                    

What is claimed is:
 1. A device for manipulating at least partiallypolarized light, comprising;a retarder stack comprising at least tworetarders, wherein at least one of the retarders in the retarder stackcomprises a first retarder that has a first orientation and a firstretardance, wherein said retarder stack receives said at least partiallypolarized light as input light and transforms said input light intotransformed light having at least a first portion with a predeterminedfirst spectrum and a first polarization and a second portion with apredetermined second spectrum and a second polarization; a modulatorthat receives at least some of said transformed light and outputsintermediate light, wherein said modulator is adapted to modulate saidfirst portion of transformed light more than said second portion oftransformed light; and at least a second retarder having a secondorientation and a second retardance that receives and transforms atleast some of said intermediate light into output light.
 2. The deviceas claimed in claim 1, wherein said first retarder, said modulator andsaid second retarder are arranged such that said modulator has at leastone state in which said partially polarized light is not modulated andhas at least one state in which a portion of said at least partiallypolarized light is modulated and the rest is not modulated.
 3. Thedevice as claimed in claim 2, wherein said modulator comprises amagneto-optic modulator.
 4. The device as claimed in claim 2, whereinsaid modulator comprises a reflection type modulator.
 5. The device asclaimed in claim 2, wherein said modulator comprises a transmission typemodulator.
 6. The device as claimed in claim 1, wherein said at leastpartially polarized light is at least partially linearly polarized witha polarization direction.
 7. The device as claimed in claim 6, whereinsaid first orientation is approximately at an angle α with respect tosaid polarization direction and said second orientation is approximatelyat an angle 90±α a with respect to said polarization direction.
 8. Thedevice as claimed in claim 6, wherein said modulator comprises aphase-mask type modulator.
 9. The device as claimed in claim 6, whereinsaid first orientation is approximately at an angle α with respect tosaid polarization direction and said second orientation is approximatelyat an angle α' with respect to said polarization direction, and whereinα and α' are different.
 10. The device as claimed in claim 6, whereinsaid first retarder, said modulator and said second retarder arearranged such that said modulator has at least one state in which saidpartially polarized light is not modulated and has at least one state inwhich a portion of said at least partially polarized light is modulatedand the rest is not modulated.
 11. The device as claimed in claim 10,wherein said modulator comprises an electro-optic modulator.
 12. Thedevice as claimed in claim 11, wherein said electro-optic modulatorcomprises a liquid crystal.
 13. The device as claimed in claim 11,wherein said electro-optic modulator comprises a nematic liquid crystal.14. The device as claimed in claim 11, wherein said electro-opticmodulator comprises one of a twisted nematic (TN), super twist nematic(STN), an electrically controlled birefringent (ECB), and a hybrid fieldeffect (HFE) material.
 15. The device as claimed in claim 11, whereinsaid electro-optic modulator comprises a surface mode device.
 16. Thedevice as claimed in claim 11, wherein said electro-optic modulatorcomprises one of a pi-cell, a zero-twist mode, a hybrid mode effect, anda polymer dispersed liquid crystal device.
 17. The device as claimed inclaim 11, wherein said electro-optic modulator comprises a surfacestabilized ferroelectric liquid crystal.
 18. The device as claimed inclaim 11, wherein said electro-optic modulator comprises a smecticliquid crystal.
 19. The device as claimed in claim 11, wherein saidelectro-optic modulator comprises a chiral smectic material.
 20. Thedevice as claimed in claim 11, wherein said electro-optic modulatorcomprises SmC*.
 21. The device as claimed in claim 11, wherein saidelectro-optic modulator comprises one of a surface stabilized FLC and avolume stabilized FLC.
 22. The device as claimed in claimed 11, whereinsaid electro-optic modulator comprises SmA*-electroclinic, distortedhelix ferroelectric, anti-ferroelectric, flexoelectric, and achiralferroelectric liquid crystal.
 23. The device as claimed in claim 1,wherein said at least partially polarized light is at least partiallycircularly polarized.
 24. The device as claimed in claim 1, wherein saidmodulator comprises a magneto-optic modulator.
 25. The device as claimedin claim 1, wherein said modulator comprises a reflection typemodulator.
 26. The device as claimed in claim 1, wherein said modulatorcomprises a transmission type modulator.
 27. The device as claimed inclaim 1, wherein said modulator comprises a polarization modulator. 28.The device as claimed in claim 27, wherein said polarization modulatoris switchable between a first and a second switching state, and has amodulation state of polarization and an isotropic state of polarization.29. A device as claimed in claim 28, wherein said second orientation andsaid second retardance are such that when said polarization modulator isin said second switching state, said output light is in the same stateof polarization.
 30. The device as claimed in claim 1, wherein saidmodulator comprises an electro-optic retarder having a fixed orientationand a retardance switchable between a first and second retardance. 31.The device as claimed in claim 30, wherein said orientation iscontinuously variable between said first and second orientations. 32.The device as claimed in claim 1, wherein said first retarder stack,said modulator and said second retarder are arranged such that saidmodulator has at least one state in which the transformed light is notmodulated and has at least one state in which one of said first andsecond portions of the transformed light is modulated and the rest isnot modulated.
 33. The device as claimed in claim 32, wherein the atleast one state in which the transformed light is not modulated is anisotropic state.
 34. The device of claim 1, wherein at least one of saidfirst and second spectra comprises a spectrum having a selectable degreeof saturation.
 35. The device of claim 1, wherein at least one of saidfirst and second spectra comprises a spectrum which is at leastapproximately 20% saturated.
 36. The device of claim 1, wherein at leastone of said first and second spectra comprises a spectrum with afull-width-half-maximum of at least 70 nm.
 37. The device of claim 1,wherein at least one of said first and second spectra comprises aspectrum which is at least approximately 70% saturated.
 38. The deviceof claim 37, wherein at least one of said first and second spectracomprises a spectrum with a full-width-half-maximum of at least 70 nm.39. A device for receiving at least partially polarized light having apolarization direction, comprising:a retarder stack comprising at leasttwo retarders a of the retarders in the retarder stack comprise a firstretarder that has a first orientation about the polarization directionand a first retardance, wherein said retarder stack receives said atleast partially polarized light as input light and transforms said inputlight into transformed light having at least a first portion with apredetermined first spectrum and a first polarization and a secondportion with a predetermined second spectrum and a second polarization;a modulator that receives at least some of said transformed light andoutputs intermediate light, wherein said modulator is adapted tomodulate said first portion of transformed light more than said portionof transformed light; and at least a second retarder having a secondorientation about the polarization direction and having a secondretardance for receiving said intermediate light and transforming saidintermediate light into output light.
 40. The device as claimed in claim39, wherein said first retarder, said modulator, and said secondretarder are arranged such that said modulator has at least one state inwhich said partially polarized light is not modulated and has at leastone state in which a portion of said at least partially polarized lightis modulated and the rest is not modulated.
 41. The device as claimed inclaim 39, wherein said first orientation is approximately at an angle αwith respect to said polarization direction and said second orientationis approximately at an angle 90±α with respect to said polarizationdirection.
 42. The device as claimed in claim 39, wherein said modulatorcomprises an electro-optic modulator.
 43. The device as claimed in claim39, wherein said modulator comprises a magneto-optic modulator.
 44. Thedevice as claimed in claim 39, wherein said modulator comprises atransmission type modulator.
 45. The device as claimed in claim 39,wherein said modulator comprises a liquid crystal.
 46. The device asclaimed in claim 39, wherein said retarder stack, said modulator andsaid second retarder are arranged such that said modulator has at leastone state in which the transformed light is not modulated and has leastone state in which one of said first and second portions of thetransformed light is modulated and the rest is not modulated.
 47. Thedevice as claimed in claim 46, wherein the at least one state in whichthe transformed light is not modulated is an isotropic state.
 48. Thedevice as claimed in claim 39, wherein said first orientation isapproximately at an angle α with respect to said polarization directionand said second orientation is approximately at an angle α' with respectto said polarization direction, and wherein α and α' are different. 49.The device of claim 39, wherein at least one of said first and secondspectra comprise a color spectrum.
 50. The device of claim 39, wherein aspectrum of said input light comprises a white light spectrum.
 51. Thedevice of claim 39, wherein said input light has an initial spectrumwhich is not a white light spectrum, and at least one of said first andsecond spectra is within said initial spectrum.
 52. The device of claim51, wherein at least one of said first and second spectra comprises acolor spectrum.
 53. The device of claim 39, wherein at least one of saidfirst and second spectra comprise a saturated spectrum.
 54. The deviceof claim 53, wherein said at least one saturated spectrum comprise colorspectrum.
 55. The device of claim 53, wherein said at least onesaturated spectrum comprises at least one primary color.
 56. The deviceof claim 53, wherein said at least one saturated spectrum comprise aband of frequencies with multiple peaks.
 57. The device of claim 53,wherein said output light comprises light having a sufficiently lowresolution to isolate a desired band of frequencies.
 58. The device ofclaim 53, wherein said at least one saturated spectrum comprises aseries of high contrast peaks.
 59. The device of claim 53, wherein saidat least one saturated spectrum comprise a band of frequencies with atleast one transmission peak with a normalized value greater than
 05. 60.The device of claim 53, wherein said at least one saturated spectrumcomprises a spectrum which is at least approximately 20% saturated. 61.The device of claim 53, wherein said at least one saturated spectrumcomprises a spectrum which is at least approximately 40% saturated. 62.The device of claim 53, wherein said at least one saturated spectrumcomprises a spectrum which is at least approximately 70% saturated. 63.The device of claim 53, wherein said at least one saturated spectrumcomprises a spectrum which is at least approximately 90% saturated. 64.The device of claim 53, wherein said retarder stack, said modulator andsaid second retarder are arranged such that said a least one saturatedspectrum exhibits a selectable degree of saturation.
 65. A device forreceiving at least partially polarized light, comprising:a retarderstack comprising at least two retarders, wherein at least one of theretarders in the retarder stack comprises a first retarder that has afirst orientation and a first retardance, wherein said retarder stackreceives said at least partially polarized light as input light andtransforms said input light into transformed light having at least afirst predetermined first spectrum and a first polarization send asecond portion with a predetermined second spectrum and a secondpolarization; a modulator that receives at least some of saidtransformed light and outputs intermediate light; and at least a secondretarder having a second orientation and a second retardance thatreceives and transforms at least some of said intermediate light intooutput light, wherein said retarder stack, said modulator, and saidsecond retarder are arranged such that said modulator has at least onestate in which neither said first spectrum nor said second spectrum aremodulated and has at least one state in which one of said at least firstspectrum and second spectrum are modulated but not the other.
 66. Thedevice as claimed in claim 65, wherein said second spectrum is acomplement of said first spectrum.
 67. The device as claimed in claim65, wherein said first retardance and said second retardance aredifferent.
 68. The device as claimed in claim 65, wherein said firstorientation is approximately perpendicular to said second orientation.69. The device as claimed in claim 65, wherein said first orientation isarranged at an angle α with respect to a given direction and said secondorientation is arranged at an angle of approximately 90±α with respectto the given direction.
 70. The device as claimed in claim 65, whereinsaid retarder stack, said modulator and said second retarder arearranged such that said modulator has at least one state in which the atleast partially polarized light is not modulated and has at least onestate in which a portion of the at least partially polarized light ismodulated and the rest is not modulated.
 71. The device as claimed inclaim 65, wherein said retarder stack, said modulator and said secondretarder are arranged such that said modulator has at least one state inwhich the transformed light is not modulated and has at least one statein which one of said first and second portions of the transformed lightis modulated and the rest is not modulated.
 72. The device as claimed inclaim 71, wherein the at least one state in which the transformed lightis not modulated is an isotropic state.
 73. The device as claimed inclaim 65, wherein said first orientation is arranged at an angle α withrespect to a given direction and said second orientation is arranged atan angle or approximately α' with respect to the given direction, andwherein α and α' are different.
 74. The device of claim 65, wherein atleast one of said first and second spectra comprises a spectrum having aselectable degree of saturation.
 75. A device for manipulating acharacteristic of at least partially polarized light, comprising:aretarder stack comprising at least two retarders, wherein at least oneof the retarders in the retarder stack comprises a first retarder thathas a first retardance and a first orientation with respect to apolarization direction of said at least partially polarized light,wherein said retarder stack receives the at least partially polarizedlight and outputs modified light having at least a first portion with apredetermined first spectrum and a first polarization and a secondportion with a predetermined second spectrum and a second polarization;a modulator that receives an least some of the modified light andoutputs intermittent light; and at least a second retarder that has asecond retardance and a second orientation with respect to saidpolarization direction of said at least partially polarized light, thatreceives the intermittent light and outputs output light, wherein saidfirst and second orientations are different, and said first retarder,said modulator, and said second retarder are arranged such that saidmodified light is not modulated by said modulator when said modulator isin one state but at least one of said first and second portions of saidmodified light is modulated by said modulator when said modulator is inanother state.
 76. The device as claimed in claim 75, wherein saidmodulator comprises an electro-optic device.
 77. A device, comprising:aretarder stack that receives input light and transforms said input lightinto transformed light having at least a first portion with apredetermined first spectrum and a first polarization and second portionwith a predetermined second spectrum and a second polarization; amodulator that receives said transformed light and that outputsintermediate light; and at least a second retarder that receives atleast some of said intermediate light and transforms said intermediatelight into output light, wherein said retarder stack, said modulator,and said second retarder are arranged such that said modulator has atleast one non-modulating state in which said transformed light and saidintermediate light are approximately the same and at least onemodulating state in which one of said first and second portions of saidtransformed light is modulated while the rest of said transformed lightis not modulated.
 78. A polarization manipulator for manipulating thepolarization distribution of light, comprising;a retarder stack thatreceives the light and that outputs modified light; a polarizationmodulator that receives the modified light and outputs intermittentlight; and at least one retarder that receives the intermittent light,manipulates the polarization distribution of the modified light, andoutputs modified output light having at least a first portion with apredetermined first spectrum and a first polarization and a secondportion with a predetermined second spectrum and a second polarizationsuch that one of said first and second portions of said modified outputlight can be selectively filtered.
 79. The polarization manipulator ofclaim 78, wherein at least one of said first and second spectracomprises a spectrum having a selectable degree of saturation.
 80. Thepolarization manipulator of claim 78, wherein at least one of said firstand second spectra comprises a spectrum which is at least approximately20% saturated.
 81. The device of claim 80, wherein at least one of saidfirst and second spectra comprises a spectrum with afull-width-half-maximum of at least 70 nm.
 82. The polarizationmanipulator of claim 78, wherein at least one of said first and secondspectra comprises a spectrum which is at least approximately 40%saturated.
 83. The polarization manipulator of claim 78, wherein atleast one of said first and second spectra comprises a spectrum which isat least approximately 70% saturated.
 84. The device of claim 83,wherein at least one of said first and second spectra comprises aspectrum with a full-width-half-maximum of at least 70 nm.
 85. Thepolarization manipulator of claim 78, wherein at least one of said firstand second spectra comprises a spectrum which is at least approximately90% saturated.
 86. The device of claim 78, wherein at least one of saidfirst and second spectra comprises a spectrum with a fullwidth-half-maximum of at least 70 nm.
 87. The device of claim 78,wherein at least one of said first and second spectra comprises aspectrum with a full-width-half-maximum of at least 97 nm.
 88. A device,comprising:a first retarder that receives input light and thattransforms said input light into transformed light which includes afirst portion with a predetermined first spectrum and a firstpolarization and a second portion with a predetermined second spectrumand second polarization; a modulator that receives said transformedlight and that outputs intermediate light; and an output retarder stackthat receives said intermediate light and that transforms saidintermediate light into output light, wherein said first retarder, saidmodulator and said second retarder stack are arranged such that saidmodulator has at least one state in which neither said first portion norsaid second portion are modulated and has at least one state in whichone of said first portion and said second portion are modulated and theother is modulated differently.
 89. The device as claimed in claim 88,wherein said output retarder stack comprises:a first output retarderhaving a first retardance and a first orientation with respect to saidat least partially polarized light that receives said intermediate lightand that outputs initially transformed light; and a second outputretarder having a second retardance and a second orientation withrespect to said at least partially polarized light that receives saidinitially transformed light and that outputs said output light, saidfirst and second orientations being different.
 90. A device,comprising:a first retarder stack that receives input light and thattransforms sais input light into transformed light which includes afirst portion with a predetermined first spectrum and a firstpolarization and a second portion with a predetermined second spectrumand a second polarization; a modulator that receives said transformedlight and that outputs intermediate light; and an output retarder thatreceives said intermediate light and that transforms said intermediatelight into output light, wherein said first retarder stack, saidmodulator and said output retarder are arranged such that said modulatorhas at least one state in which neither said first portion nor saidsecond portion are modulated and has at least one state in which one ofsaid first portion and said second portion is modulated and the other ismodulated differently.
 91. The device as claimed in claim 90, whereinsaid first retarder stack comprises:a first retarder having a firstretardance and a first orientation with respect to an at least partiallypolarized light, that receives said at least partially polarized lightand that outputs initially transformed light; and a second retarderhaving a second retardance and a second orientation with respect to saidat least partially polarized light that receives said initiallytransformed light and that output said transformed light said first andsecond orientations being different.
 92. A device, comprising:a firstretarder stack that receives input light and that transforms said inputlight into transformed light which includes a first portion with apredetermined first spectrum and a first polarization and a secondportion with a predetermined second spectrum and a second polarization;a modulator that receives said transformed light and that outputsintermediate light; and a second retarder stack that receives saidintermediate light and that transformed said intermediate light intooutput light, wherein said first retarder stack, said modulator, andsaid second retarder stack are arranged such that said modulator has atleast one state in which neither said first portion nor said secondportion are modulated and has at least one state in which one of saidfirst portion and said second portion is modulated and the other ismodulated differently.
 93. The device as claimed in claim 92, whereinsaid first retarder stack comprises:a first retarder having a firstretardance and a first orientation with respect to an at least partiallypolarized light, that receives said at least partially polarized lightand that outputs initially transformed light; and a second retarderhaving a second retardance and a second orientation with respect to saidat least partially polarized light that receives said initiallytransformed light and that outputs said transformed light, said firstand second orientations being different.
 94. The device as claimed inclaim 92, wherein said second retarder stack comprises:a first retarderhaving a first retardance and a first orientation with respect to an atleast partially polarized light, that receives said intermediate lightand that outputs initially transformed light; and a second retarderhaving a second retardance and a second orientation with respect to saidat least partially polarized light that receives said initiallytransformed light and that outputs said output light, said first andsecond orientations being different.
 95. The device as claimed in claim92, wherein said first retarder stack and said second retarder stack arearranged such that the polarization of the first portion and the secondportion are different.
 96. The device as claimed in claim 92, whereinsaid first spectrum and said second spectrum are complements of eachother.
 97. The device as claimed in claim 96, wherein said firstretarder stack and said second retarder stack are arranged such that thepolarization of the first portion and the polarization of the secondportion are not equal.
 98. A device, comprising:a first retarder stackthat receives input light and that transforms said input light intotransformed light which includes a first portion with a predeterminedfirst spectrum and a first polarization and a second portion with apredetermined second spectrum and a second polarization; a modulatorthat receives said intermediate light and that outputs intermediatelight; and a second retarder stack that receives said intermediate lightand that transforms said intermediate light into output light, whereinsaid first retarder stack, said modulator, and second retarder stack arearranged such that said modulator has at least one non-modulating statein which said transformed light and said intermediate light areapproximately the same and at least one modulating state in which acharacteristic of one of said first portion and said second portion ofsaid transformed light is modulated.
 99. The device as claimed in claim98, wherein said first retarder stack, said modulator, and said secondretarder stack are arranged such that the polarization of said modulatedportion of said transformed light is modulated.
 100. The device asclaimed in claim 98, wherein said characteristic is a polarizationdistribution of said modulated portion of said transformed light. 101.The device as claimed in claim 98, wherein said characteristic is aphase distribution of said modulated portion of said transformed light.102. The device as claimed in claim 98, wherein said first stackcomprises a first plurality of retarders, and said second stackcomprises a second plurality of retarders.
 103. The device as claimed inclaim 102, wherein the retardances and orientations of said first stackand said second stack follow a Fan olc design.
 104. The device asclaimed in claim 102, wherein the retardances and orientations of saidfirst plurality of retarders and said second plurality of secondretarders follow a quasi-folded olc design.
 105. The device as claimedin claim 102, wherein the retardances and orientations of said firstplurality of retarders and said second plurality of second retardersfollow a split-element design.
 106. A polarization manipulator to modifya polarization distribution of light, comprising:a first plurality ofretarders having respective first plurality of orientations α_(N) and arespective first plurality of retardances Γ_(N), that manipulates thepolarization distribution of the light to yield polarization manipulatedlight having a first portion with a predetermined first spectrum and afirst polarization and a second portion with a predetermined secondspectrum and a second polarization; a polarization modulating devicethat receives the polarization modified light and that outputsintermediate light, wherein said polarization modulating device isadapted to modulate said first portion of polarization manipulated lightmore than said second portion of polarization manipulated light; and asecond plurality of retarders having a respective second plurality oforientations α'_(N) and a respective second plurality of retardancesΓ'_(N) that receives and manipulates the polarization distribution ofthe intermediate light and that outputs modified output light having amodified polarization distribution for color selection.
 107. Thepolarization manipulator as claimed in claim 106, further comprising ananalyzer for receiving said light with the modified polarizationdistribution for color selection.
 108. The polarization manipulator asclaimed in claim 106, wherein α'_(N) =90±α_(N).
 109. A spectral filterfor use with at least partially polarized light, comprising one or morewhite/filtered stages in series, each stage comprising:a modulatorswitchable between a first and second switching state, and having amodulation state, and having a modulation state of polarization and anisotropic state of polarization; a first retarder stack, positioned on afirst side of said modulator, comprising two or more retarders, whereinthe number, N, of said retarders and the retardances and orientations ofsaid retarders are such that a first spectrum is transmitted in saidmodulation state of polarization and a second, complementary, spectrumis transmitted in said isotropic state of polarization; and a secondretarder stack, positioned on the opposite side of said modulator,comprising N retarders, wherein the retardances and orientations of saidretarders are such that in said first switching state said first andsecond spectra are transmitted in orthogonal states of polarization.110. The filter of claim 109 wherein the retardances and orientations ofsaid retarders in said second stack are such that in said secondswitching state said first and second spectra are transmitted in thesame state of polarization.
 111. The filter of claim 109 furthercomprising an analyzing polarizer in series with said stages.
 112. Thefilter of claim 111 further comprising an input polarizer in series withsaid stages.
 113. The filter of claim 111 wherein said analyzingpolarizer is oriented at 0°.
 114. The filter of claim 113 wherein saidlinear polarizer comprises a circular polarizer in combination with aquarter-wave retarder.
 115. The filter of claim 111 wherein saidanalyzing polarizer is oriented at 90°.
 116. The filter of claim 115wherein said polarizer is selected from the group consisting ofabsorption, dichroic, dye-based, non-absorptive, polarizing dielectricfilm, polarizing beamsplitter, calcite, quartz, scattering, prismaticpolarizers.
 117. The filter of claim 111 wherein said analyzingpolarizer is a linear polarizer.
 118. The filter of claim 111 whereinsaid analyzing polarizer is a circular polarizer.
 119. The filter ofclaim 118 wherein said circular polarizer is a cholesteric LC polarizer.120. The filter of claim 109 wherein said modulator is an electro-opticmodulator and comprises a retarder having a fixed orientation and aretardance switchable between a first and a second retardance.
 121. Thefilter of claim 120 wherein said orientation is 0° and said firstretardance is zero retardance and said second retardance is half-waveretardance.
 122. The filter of claim 120 wherein said retardance iscontinuously variable between said first and second retardances. 123.The filter of claim 120 wherein said retarder comprises a nematic liquidcrystal retarder.
 124. The filter of claim 109 wherein said modulatorcomprises a retarder selected from the group consisting of twistednematic, super twist nematic, electrically controlled birefringence,hybrid field effect, surface mode, pi-cell, zero-twist mode, hybrid modeand polymer dispersed liquid crystal retarders.
 125. The filter of claim124 wherein said retarder is a chiral smectic A* nematic.
 126. Thefilter of claim 124 wherein said retarder is a distorted helixferroelectric.
 127. The filter of claim 124 wherein said retarder isanalog SmC*.
 128. The filter of claim 109 wherein said modulatorcomprises a retarder having a fixed retardance and an orientationswitchable between a first and a second orientation.
 129. The filter ofclaim 128 wherein said fixed retardance is a half-wave retardance. 130.The filter of claim 129 wherein said first orientation is 0° and saidsecond orientation is 45°.
 131. The filter of claim 128 wherein saidorientation is continuously rotatable between said first and secondvalues.
 132. The filter of claim 128 wherein said retarder is a smecticliquid crystal retarder.
 133. The filter of claim 128 wherein saidmodulator further comprises first and second quarter-wave retarderspositioned on either side of said retarder.
 134. The filter of claim 109wherein said modulator comprises a retarder selected from the groupconsisting of chiral smectic, ferroelectric, SmC*, surface stabilizedSmC*, volume stabilized SmC*, binary SmC*, analog SmC*, SmA*, distortedhelix ferroelectric, anti-ferroelectric, flexoelectric, and achiralferroelectric liquid crystal retarders.
 135. The filter of claim 134wherein said retarder is a chiral smectic A*.
 136. The filter of claim134 wherein said retarder is a distorted helix ferroelectric.
 137. Thefilter of claim 134 wherein said retarder is analog SmC*.
 138. Thefilter of claim 109 wherein said modulator comprises a compoundretarder.
 139. The filter of claim 138 wherein said compound retarder isan achromatic compound retarder.
 140. The filter of claim 138 whereinsaid compound retarder comprises a liquid crystal retarder and a passiveretarder.
 141. The filter of claim 140 wherein said modulator comprisesa magneto-optic modulator.
 142. The filter of claim 109 wherein thedesign wavelength of said electro-optic modulator is within said firstspectrum.
 143. The filter of claim 142 wherein the additive primariesare centered at λ=450 nm, λ=550 nm and λ=590.
 144. The filter of claim109 wherein the retardances and orientations of said retarders in saidsecond stack are such that said filter is normally white.
 145. Thefilter of claim 144 wherein said retarders in said first stack haveretardances Γ₁, Γ₂ . . . Γ_(N) and orientations α₁, α₂ . . . α_(N), andsaid retarders in said second stack have retardances Γ_(N) . . . Γ₂, Γ₁and orientations 90+α_(N) . . . 90+α₂, 90+α₁.
 146. The filter of claim109 wherein the retardances and orientations of said retarders in saidsecond stack are such that said filter is normally filtered.
 147. Thefilter of claim 146 wherein said retarders in said first stack haveretardances Γ₁, Γ₂ . . . Γ_(N) and orientations α₁, α₂ . . . α_(N), andsaid retarders in said second stack have retardances Γ_(N) . . . Γ₂, Γ₁and orientations 90-α_(N) . . . 90-α₂, 90-α₁.
 148. The filter of claim109 wherein said first spectrum is an additive primary spectrum and saidsecond spectrum is a subtractive primary spectrum.
 149. The filter ofclaim 109 wherein said first spectrum is a subtractive primary spectrumand said second spectrum is an additive primary spectrum.
 150. Thefilter of claim 109 wherein said filter has first and second stages.151. The filter of claim 109 wherein said filter has first, second andthird stages.
 152. The filter of claim 151 wherein the first and secondspectra of said first stage, the first and second spectra of said secondstage and the first and second spectra of said third stage are such thatthe filter output is switchable between a first primary spectrum, asecond primary spectrum and a third primary spectrum.
 153. The filter ofclaim 152 wherein said first, second and third primary spectra areadditive primary spectra.
 154. The filter of claim 152 wherein saidfilter output is further switchable to white.
 155. The filter of claim151 further comprising an input polarizer and an analyzing polarizer butno internal polarizers between said stages.
 156. The filter of claim 151further comprising an input polarizer, an analyzing polarizer, a firstinternal polarizer between said first and second stages, and a secondinternal polarizer between said second and third stages.
 157. A methodof providing red, green, and blue filtering, comprising the stepsof:providing the filter of claim 151; and switching the modulators ofsaid first, second and third stages between said first and secondswitching states.
 158. A multipixel filter comprising a plurality of thefilters of claim
 109. 159. A camera comprising the multipixel filter ofclaim 158 and further comprising a receiver.
 160. The camera of claim159 wherein said camera is a still camera.
 161. The camera of claim 160wherein said camera is a video camera.
 162. A display comprising themultipixel filter of claim 158 and further comprising light source. 163.The display of claim 162 wherein said display is a projection display.164. The display of claim 162 wherein said display is a direct viewdisplay.
 165. The filter of claim 109 adapted for use with an opticaladdressing signal and further including a photodetector coupled to saidfilter.
 166. A method of obtaining light switchable between white andfiltered, comprising the steps of:providing the filter of claim 109; andswitching said modulator between said first and second switching states.167. The method of claim 166 wherein said switching is analog switching.168. A spectral filter for use with polarized light, comprising one ormore white/filtered stages in series, each stage comprising:apolarization modulator; a first retarder stack, positioned on a firstside of said modulator, comprising N retarders, where N is two or more,said retarders having retardances Γ₁, Γ₂ . . . Γ_(N) and orientationsα₁, α₂ . . . α_(N) ; and a second retarder stack, positioned on theopposite side of said electro-optic modulator, comprising N retarders,said retarders having retardances Γ_(N) . . . Γ₂, Γ₁ and orientations90±α_(N) . . . 90±α₂, 90±α₁.
 169. The filter of claim 168 wherein N≧1.170. The filter of claim 168 wherein N=2 and Γ₁ =Γ₂.
 171. The filter ofclaim 168 wherein N=2 and Γ₁ =2Γ₂.
 172. The filter of claim 168 whereinN=2 and Γ₂ =2Γ₁.
 173. The filter of claim 168 wherein the retardancesand orientations of said retarders follow a fan olc design.
 174. Thefilter of claim 173 wherein α_(N) =(2N-1)α₁.
 175. The filter of claim173 wherein N=1 and α₁ ≈22°.
 176. The filter of claim 174 wherein N=2,Γ₁ =Γ₂ and α₁ ≈11°.
 177. The filter of claim 173 wherein N=2, Γ₁ =2Γ₂and α₁ ≈15°.
 178. The filter of claim 173 wherein N=3, Γ₁ =Γ₂₌Γ 3 and α₁≈7°.
 179. The filter of claim 173 wherein N=3, Γ₁ =Γ₂₌₂Γ 3 and α₁ ≈9°.180. The filter of claim 168 wherein the retardances and orientations ofsaid retarders follow a quasi-folded olc design.
 181. The filter ofclaim 180 wherein α_(N) =(-1)^(N+1) α₁.
 182. The filter of claim 181wherein N=2, Γ₁ =Γ₂ and α₁ ≈11°.
 183. The filter of claim 181 whereinN=3, Γ₁ =Γ₂₌Γ 3 and α₁ ≈8°.
 184. The filter of claim 181 wherein N=3, Γ₁=Γ₂₌Γ 3 and α₁ ≈15°.
 185. The filter of claim 181 wherein N=3, Γ₁ =Γ₂₌Γ3 and α₁ ≈22°.
 186. The filter of claim 168 wherein the retardances andorientations of said retarders follow a split-element design.
 187. Thefilter of claim 186 wherein N=1 and α₁ =45°.
 188. The filter of claim187 further comprising a center retarder, positioned between said firstretarder stack and said modulator, oriented at α_(C) =0° and havingretardance Γ_(C), where Γ₁ =Γ_(C) +π/2.
 189. The filter of claim 186wherein N=2, α₁ =45°, α₂ =0° and Γ₁ =2Γ₂ +π/2.
 190. The filter of claim168 wherein said retarders in said second stack have orientations90+α_(N) . . . 90+α₂, 90+α₁.
 191. The filter of claim 168 wherein saidretarders in said second stack have orientations 90-α_(N) . . . 90-α₂,90-α₁.
 192. The filter of claim 168 further comprising an analyzingpolarizer in series with said stages.
 193. The filter of claim 192further comprising an input polarizer in series with said stages. 194.The filter of claim 193 wherein said analyzing polarizer is oriented at0°.
 195. The filter of claim 193 wherein said analyzing polarizer is acircular polarizer.
 196. The filter of claim 195 wherein said circularpolarizer is a cholesteric circular polarizer.
 197. The filter of claim192 wherein said analyzing polarizer is a neutral linear polarizer. 198.The filter of claim 197 wherein said polarizer is a linear polarizercomprise a circular polarizer in combination with quarter-wave retarder.199. The filter of claim 197 wherein said linear polarizer is a EMIndustries, Inc. Transmax polarizer.
 200. The filter of claim 168wherein said polarization modulator comprises an electro-optic retarderhaving a fixed orientation and a retardance switchable between a firstand a second retardance.
 201. The filter of claim 200 wherein saidorientation is 0° and said first retardance is zero retardance and saidsecond retardance is half-wave retardance.
 202. The filter of claim 200wherein said retardance is continuously variable between said first andsecond retardances.
 203. The filter of claim 200 wherein said retardercomprises a nematic liquid crystal retarder.
 204. The filter of claim203 wherein said retarder is comprises one of a TN or STN GWface modeIT-cell ECB, and HFE.
 205. The filter of claim 168 wherein saidmodulator comprises an LC retarder having a fixed retardance and anorientation switchable between a first and a second orientation. 206.The filter of claim 205 wherein said fixed retardance is a half-waveretardance.
 207. The filter of claim 206 wherein said electro-opticmodulator further comprises first and second quarter-wave retarderspositioned on either side of said retarder.
 208. The filter of claim 205wherein said orientation is continuously rotatable between said firstand second orientations.
 209. The filter of claim 205 wherein saidretarder is a smectic liquid crystal retarder.
 210. The filter of claim209 wherein said retarder comprises one of a chiral smectic, SMA*, DHF,SmC*, and antiferroelectric.
 211. The filter of claim 168 wherein saidelectro-optic modulator comprises a compound retarder.
 212. The filterof claim 211 wherein said compound retarder comprises a liquid crystalretarder and a passive retarder.
 213. The filter of claim 168 wherein N,Γ₁, Γ₂ . . . Γ_(N), and α₁, α₂ . . . α_(N) are such that a firstspectrum is transmitted in a first state of polarization and a second,complementary, spectrum is transmitted in the orthogonal state ofpolarization.
 214. The filter of claim 213 wherein the design wavelengthof said electro-optic modulator is within said first spectrum.
 215. Thefilter of claim 213 wherein said first spectrum is an additive primaryspectrum and said second spectrum is a subtractive primary spectrum.216. The filter of claim 168 wherein said filter has two stages. 217.The filter of claim 168 wherein said filter comprises at least threestages.
 218. The filter of claim 217 wherein N, Γ₁, Γ₂ . . . Γ_(N), andα₁, α₂ . . . α_(N) for each stage are such that the filter output isswitchable between a first primary spectrum, a second primary spectrumand a third primary spectrum.
 219. The filter of claim 168 wherein saidfirst, second and third primary spectra are additive primary spectra.220. The filter of claim 219 wherein said filter output is furtherswitchable to white.
 221. A polarization device to modify thepolarization of an at least partially polarized light, comprising:aplurality of cascaded polarization manipulators, each including:aretarder stack comprising at least two retarders, wherein at least oneof the retarders in the retarder stack comprises a first retarder thathas a first orientation and a first retardance that receives an inputlight and that transforms the input light into transformed light havingat least a first portion with a predetermined first spectrum and a firstpolarization and a second portion with a predetermined second spectrumand a second polarization; a modulator, coupled to said retarder stack,that receives the transformed light and that outputs intermediate light,wherein said modulator is adapted to modulate said first portion oftransformed light more than said second portion of transformed light;and a second retarder, coupled to said modulator, having a secondorientation and a second retardance that receives the intermediate lightand that transforms the intermediate light into output light.
 222. Thepolarization device of claim 221, wherein said retarder stack, saidmodulator and said second retarder are arranged such that said modulatorhas at least one state in which the transformed light is not modulatedand has at least one state in which one of said first and secondportions of the transformed light is modulated and the rest is notmodulated.
 223. The polarization device of claim 222, wherein saidplurality of cascaded polarization manipulators includes at least afirst, a second, and a third polarization manipulator, and wherein saidfirst, second, and third polarization manipulators each has a firststate characterized by selectability of a first, second, and thirdcolor, respectively, and a second state characterized by selectabilityof white.
 224. The polarization device of claim 223 wherein the first,second, and third color are red, green, and blue, respectively.
 225. Thepolarization device of claim 223, wherein the first, second, and thirdcolor are cyan, magenta, and yellow, respectively.
 226. The polarizationdevice of claim 221, wherein the input light is at least partiallylinearly polarized with a polarization direction.
 227. The polarizationdevice of claim 226, wherein said first orientation is approximately atan angle α_(N) with respect to said polarization direction and saidsecond orientation is approximately at an angle 90±α_(N) with respect tosaid polarization direction.
 228. The polarization device of claim 226wherein said first orientation is arranged at an angle α_(N) withrespect to a given direction and said second orientation is arranged atan angle of approximately α_(N) ' with respect to the given direction,and wherein α_(N) and α_(N) ' are different.
 229. The polarizationdevice of claim 221, wherein each of the cascaded polarizationmanipulators further comprises:an analyzer, coupled to said secondretarder, for receiving the output light and outputting analyzed outputlight.
 230. The polarization device of claim 221, further comprising apolarizer, coupled to a first polarization manipulator of said pluralityof cascaded polarization manipulators, for receiving a received lightand outputting the at least partially polarized light.
 231. Thepolarization device of claim 230, wherein the received light isunpolarized.
 232. A polarization device to modify the polarization of anat least partially polarized light, comprising;a plurality of cascadedpolarization manipulators, each including:a first plurality of retardershaving a respective first plurality of orientations α_(N) and arespective first plurality of retardances Γ_(N), that manipulates apolarization distribution of a portion of an input light to yieldtransformed light having a first portion with a predetermined firstspectrum and a first polarization and a second portion with apredetermined second spectrum and a second polarization; a polarizationmodulating device, coupled to said first plurality of retarders, thatreceives the transformed light and that outputs intermediate light,wherein said polarization modulating device is adapted to modulate saidfirst transformed light more than said second portion of transformedlight; and a second plurality of retarders, coupled to said polarizationmodulating device, having a respective second plurality of orientationsα'_(N) and a respective second plurality of retardances Γ'_(N) thatreceives and manipulates polarization distribution of a portion of theintermediate light and outputs light.
 233. The polarization device ofclaim 232, wherein said first plurality of retarders, said polarizationmodulating device, and said second plurality of retarders are arrangedsuch that each polarization modulating device has least one state inwhich the transformed light is not modulated and has at least one statein which one of said first and second portions of the transformed lightis modulated and the rest is not modulated.
 234. The polarization deviceof claim 232, wherein α'_(N) is approximately equal to 90±α_(N). 235.The polarization device of claim 232, wherein α_(N) and α_(N) ' aredifferent.
 236. The polarization device of claim 232, wherein saidplurality of cascaded polarization manipulators includes at least afirst, a second, and a third polarization manipulator, and wherein saidfirst, second, and third polarization manipulators each has a firststate characterized by selectability of a first, second, and thirdcolor, respectively, and a second state characterized by selectabilityof white.
 237. The polarization device of claim 236, wherein the first,second, and third color are red, green, and blue, respectively.
 238. Thepolarization device of claim 236, wherein the first, second, and thirdcolor are cyan, magenta, and yellow, respectively.
 239. The polarizationdevice of claim 236, wherein each of the cascaded polarizationmanipulators further comprises:an analyzer, coupled to said secondplurality of retarders, for receiving the output light and outputtinganalyzed output light.
 240. The polarization device of claim 232,further comprising a polarizer, coupled to a first polarizationmanipulator of said plurality of cascaded polarization manipulators, forreceiving a received light and outputting the at least partiallypolarized light.
 241. The polarization device of claim 240, wherein thereceived light is unpolarized.
 242. A device for receiving at leastpartially polarized light having a polarization direction, comprising:afirst retarder stack comprising at least two retarders, including afirst retarder and a second retarder, said first retarder having a firstretardance and a first orientation with respect to the polarizationdirection of said at least partially polarized light, said secondretarder having a second retardance and a second orientation withrespect to the polarization direction of said at least partiallypolarized light; a modulator; and a second retarder stack comprising atleast two retarders, including a third retarder having a thirdretardance and a third orientation with respect to the polarizationdirection of said at least partially polarized light, and a fourthretarder having a fourth retardance and a fourth orientation withrespect to the polarization direction of said at least partiallypolarized light, wherein said first retarder, said second retarder, saidmodulator, said third retarder and said fourth retarder are opticallycoupled to each other and arranged such that said at least partiallypolarized light is transformed into output light having at least a firstportion with a predetermined first spectrum and a first polarization anda second portion having a predetermined second spectrum and a secondpolarization.
 243. The device of claim 242, wherein at least one of saidfirst and second spectra comprises a spectrum having a selectable degreeof saturation.
 244. The device of claim 242, wherein one of said firstand second spectra comprises a spectrum which is at least approximately20 percent saturated.
 245. The device of claim 244, wherein at least oneof said first and second spectra comprises a spectrum with afull-width-half-maximum of at least 70 nm.
 246. The device of claim 242,wherein one of said first and second spectra comprises a spectrum whichis at least approximately 40 percent saturated.
 247. The device of claim246, wherein at least one of said first and second spectra comprises aspectrum with a full-width-half-maximum of at least 70 nm.
 248. Thedevice of claim 242, wherein one of said first and second spectracomprises a spectrum which is at least approximately 70 percentsaturated.
 249. The device of claim 248, wherein at least one of saidfirst and second spectra comprises a spectrum with afull-width-half-maximum of at least 70 nm.